diff --git a/.appveyor.yml b/.appveyor.yml
index 3e05fd5c4..7d74c1d9a 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -3,6 +3,10 @@ version: '{build}'
 # use an image with recent Mingw-w64 versions available on both architectures: https://www.appveyor.com/docs/windows-images-software/#mingw-msys-cygwin
 image: Visual Studio 2015
 
+environment:
+  # disable LFS file downloading during regular cloning
+  GIT_LFS_SKIP_SMUDGE: 1
+
 cache:
   - NimBinaries
 
diff --git a/.travis.yml b/.travis.yml
index 9828e5489..de7cc25bb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,6 +9,8 @@ cache:
 git:
   # when multiple CI builds are queued, the tested commit needs to be in the last X commits cloned with "--depth X"
   depth: 10
+  # disable LFS file downloading during regular cloning
+  lfs_skip_smudge: true
 
 matrix:
   include:
diff --git a/Makefile b/Makefile
index d9d73402b..8ef08864d 100644
--- a/Makefile
+++ b/Makefile
@@ -28,15 +28,6 @@ ENV_SCRIPT := "$(CURDIR)/env.sh"
 # duplicated in "env.sh" to prepend NIM_DIR/bin to PATH
 NIM_DIR := vendor/Nim
 
-#- forces a rebuild of csources, Nimble and a complete compiler rebuild, in case we're called after pulling a new Nim version
-#- uses our Git submodules for csources and Nimble (Git doesn't let us place them in another submodule)
-#- build_all.sh looks at the parent dir to decide whether to copy the resulting csources binary there,
-#  but this is broken when using symlinks, so build csources separately (we get parallel compiling as a bonus)
-#- Windows is a special case, as usual
-#- macOS is also a special case, with its "ln" not supporting "-r"
-#- the AppVeyor 32-build is done on a 64-bit image, so we need to override the architecture detection with ARCH_OVERRIDE
-BUILD_NIM := echo -e $(BUILD_MSG) "Nim compiler" && \
-	V=$(V) CC=$(CC) MAKE=$(MAKE) ARCH_OVERRIDE=$(ARCH_OVERRIDE) "$(CURDIR)/build_nim.sh" "$(NIM_DIR)" ../Nim-csources ../nimble
 ifeq ($(OS), Windows_NT)
   EXE_SUFFIX := .exe
 else
@@ -49,7 +40,7 @@ ifeq ($(shell uname), Darwin)
 else
   MD5SUM := md5sum
 endif
-# AppVeyor cache of $(NIM_DIR)/bin that doesn't get unpacked after all submodules have been checked out, like on Travis
+# AppVeyor/Travis cache of $(NIM_DIR)/bin
 CI_CACHE :=
 
 # debugging tools + testing tools
@@ -154,33 +145,34 @@ github-ssh:
 	git config url."git@github.com:".insteadOf "https://github.com/"
 	git submodule foreach --recursive 'git config url."git@github.com:".insteadOf "https://github.com/"'
 
-#- re-builds the Nim compiler (not usually needed, because `make update` does it when necessary)
+#- conditionally re-builds the Nim compiler (not usually needed, because `make update` calls this rule; delete $(NIM_BINARY) to force it)
 #- allows parallel building with the '+' prefix
-build-nim: | deps
-	+ $(BUILD_NIM)
+#- forces a rebuild of csources, Nimble and a complete compiler rebuild, in case we're called after pulling a new Nim version
+#- uses our Git submodules for csources and Nimble (Git doesn't let us place them in another submodule)
+#- build_all.sh looks at the parent dir to decide whether to copy the resulting csources binary there,
+#  but this is broken when using symlinks, so build csources separately (we get parallel compiling as a bonus)
+#- Windows is a special case, as usual
+#- macOS is also a special case, with its "ln" not supporting "-r"
+#- the AppVeyor 32-build is done on a 64-bit image, so we need to override the architecture detection with ARCH_OVERRIDE
+build-nim: | sanity-checks
+	+ NIM_BUILD_MSG="$(BUILD_MSG) Nim compiler" \
+		V=$(V) \
+		CC=$(CC) \
+		MAKE=$(MAKE) \
+		ARCH_OVERRIDE=$(ARCH_OVERRIDE) \
+		"$(CURDIR)/build_nim.sh" "$(NIM_DIR)" ../Nim-csources ../nimble "$(CI_CACHE)"
 
 #- initialises and updates the Git submodules
 #- manages the AppVeyor cache of Nim compiler binaries
 #- deletes the ".nimble" dir to force the execution of the "deps" target
 #- deletes and recreates "nimbus.nims" which on Windows is a copy instead of a proper symlink
 #- allows parallel building with the '+' prefix
-#- rebuilds the Nim compiler after the corresponding submodule is updated (keep in mind that Git doesn't preserve file timestamps)
+#- rebuilds the Nim compiler if the corresponding submodule is updated
 $(NIM_BINARY) update: | sanity-checks
 	git submodule update --init --recursive
-	[[ -n "$(CI_CACHE)" && -d "$(CI_CACHE)" ]] && \
-		cp -a "$(CI_CACHE)"/* $(NIM_DIR)/bin/ || \
-		true
 	rm -rf $(NIMBLE_DIR) nimbus.nims && \
 		$(MAKE) nimbus.nims
-	+ "$(CURDIR)/nim_needs_rebuilding.sh" "$(NIM_DIR)" $(NIM_BINARY) && \
-		{ \
-			$(BUILD_NIM); \
-			[[ -n "$(CI_CACHE)" ]] && \
-			rm -rf "$(CI_CACHE)" && \
-			mkdir $(CI_CACHE) && \
-			cp -a $(NIM_DIR)/bin/* "$(CI_CACHE)"/ || \
-			true; \
-		} || true
+	+ $(MAKE) build-nim
 
 # don't use this target, or you risk updating dependency repos that are not ready to be used in Nimbus
 update-remote:
diff --git a/README.md b/README.md
index 6d92d59b4..900e2b502 100644
--- a/README.md
+++ b/README.md
@@ -148,12 +148,19 @@ make LOG_LEVEL=TRACE nimbus # log everything
 make NIMFLAGS="-d:release"
 ```
 
-- if you want to use SSH keys with GitHub:
+- if you want to use SSH keys with GitHub (also handles submodules):
 
 ```bash
 make github-ssh
 ```
 
+- force a Nim compiler rebuild:
+
+```bash
+rm vendor/Nim/bin/nim
+make -j8 build-nim
+```
+
 #### Git submodule workflow
 
 Working on a dependency:
diff --git a/build_nim.sh b/build_nim.sh
index 1bc287be9..b56fe3869 100755
--- a/build_nim.sh
+++ b/build_nim.sh
@@ -7,77 +7,120 @@ CSOURCES_COMMIT="b56e49bbedf62db22eb26388f98262e2948b2cbc" # 0.19.0
 NIMBLE_COMMIT="c8d79fc0228682677330a9f57d14389aaa641153" # Mar 26 10:06:06 2019
 
 # script arguments
-[[ $# -ne 3 ]] && { echo "usage: $0 nim_dir csources_dir nimble_dir"; exit 1; }
+[[ $# -ne 4 ]] && { echo "usage: $0 nim_dir csources_dir nimble_dir ci_cache_dir"; exit 1; }
 NIM_DIR="$1"
 CSOURCES_DIR="$2"
 NIMBLE_DIR="$3"
+CI_CACHE="$4"
 
 ## env vars
 # verbosity level
 [[ -z "$V" ]] && V=0
-[[ "$V" == "0" ]] && exec &>/dev/null
 [[ -z "$CC" ]] && CC="gcc"
 # to build csources in parallel, set MAKE="make -jN"
 [[ -z "$MAKE" ]] && MAKE="make"
 # for 32-bit binaries on a 64-bit Windows host
 UCPU=""
 [[ "$ARCH_OVERRIDE" == "x86" ]] && UCPU="ucpu=i686"
+[[ -z "$NIM_BUILD_MSG" ]] && NIM_BUILD_MSG="Building the Nim compiler"
 
 # Windows detection
-ON_WINDOWS=0
-uname | grep -qi mingw && ON_WINDOWS=1
+if uname | grep -qi mingw; then
+	ON_WINDOWS=1
+	EXE_SUFFIX=".exe"
+else
+	ON_WINDOWS=0
+	EXE_SUFFIX=""
+fi
 
-# working directory
-cd "$NIM_DIR"
+NIM_BINARY="${NIM_DIR}/bin/nim${EXE_SUFFIX}"
 
-# Git repos for csources and Nimble
-[[ -d "$CSOURCES_DIR" ]] || { \
-	mkdir -p "$CSOURCES_DIR" && \
-	cd "$CSOURCES_DIR" && \
-	git clone https://github.com/nim-lang/csources.git . && \
-	git checkout $CSOURCES_COMMIT && \
-	cd - >/dev/null; \
-}
-[[ "$CSOURCES_DIR" != "csources" ]] && \
-rm -rf csources && \
-ln -s "$CSOURCES_DIR" csources
+nim_needs_rebuilding() {
+	REBUILD=0
+	NO_REBUILD=1
 
-# we have to delete .git or koch.nim will checkout a branch tip
-[[ -d "$NIMBLE_DIR" ]] || { \
-	mkdir -p "$NIMBLE_DIR" && \
-	cd "$NIMBLE_DIR" && \
-	git clone https://github.com/nim-lang/nimble.git . && \
-	git checkout $NIMBLE_COMMIT && \
-	rm -rf .git && \
-	cd - >/dev/null; \
-}
-[[ "$NIMBLE_DIR" != "dist/nimble" ]] && \
-mkdir -p dist && \
-rm -rf dist/nimble && \
-ln -s ../"$NIMBLE_DIR" dist/nimble
+	if [[ -n "$CI_CACHE" && -d "$CI_CACHE" ]]; then
+		if [[ ! -e "$NIM_DIR" ]]; then
+			git clone --depth=1 https://github.com/status-im/Nim.git "$NIM_DIR"
+		fi
+		cp -a "$CI_CACHE"/* "$NIM_DIR"/bin/
+	fi
 
-# bootstrap the Nim compiler and build the tools
-rm -rf bin/nim_csources && \
-cd csources && { \
-	[[ "$ON_WINDOWS" == "0" ]] && { \
-		$MAKE clean && \
-		$MAKE LD=$CC; \
-	} || { \
-		$MAKE myos=windows $UCPU clean && \
-		$MAKE myos=windows $UCPU CC=gcc LD=gcc; \
-	}; \
-} && \
-cd - >/dev/null && { \
-	[ -e csources/bin ] && { \
-		cp -a csources/bin/nim bin/nim && \
-		cp -a csources/bin/nim bin/nim_csources && \
-		rm -rf csources/bin; \
-	} || { \
-		cp -a bin/nim bin/nim_csources; \
-	}; \
-} && { \
-	sed 's/koch tools/koch --stable tools/' build_all.sh > build_all_custom.sh; \
-	sh build_all_custom.sh; \
-	rm build_all_custom.sh; \
+	# compare binary mtime to the date of the last commit (keep in mind that Git doesn't preserve file timestamps)
+	if [[ -e "$NIM_BINARY" && $(stat -c%Y "$NIM_BINARY") -gt $(cd "$NIM_DIR"; git log --pretty=format:%cd -n 1 --date=unix) ]]; then
+		return $NO_REBUILD
+	else
+		return $REBUILD
+	fi
 }
 
+build_nim() {
+	echo -e "$NIM_BUILD_MSG"
+	[[ "$V" == "0" ]] && exec &>/dev/null
+
+	# working directory
+	cd "$NIM_DIR"
+
+	# Git repos for csources and Nimble
+	if [[ ! -d "$CSOURCES_DIR" ]]; then
+		mkdir -p "$CSOURCES_DIR"
+		cd "$CSOURCES_DIR"
+		git clone https://github.com/nim-lang/csources.git .
+		git checkout $CSOURCES_COMMIT
+		cd - >/dev/null
+	fi
+	if [[ "$CSOURCES_DIR" != "csources" ]]; then
+		rm -rf csources
+		ln -s "$CSOURCES_DIR" csources
+	fi
+
+	if [[ ! -d "$NIMBLE_DIR" ]]; then
+		mkdir -p "$NIMBLE_DIR"
+		cd "$NIMBLE_DIR"
+		git clone https://github.com/nim-lang/nimble.git .
+		git checkout $NIMBLE_COMMIT
+		# we have to delete .git or koch.nim will checkout a branch tip, overriding our target commit
+		rm -rf .git
+		cd - >/dev/null
+	fi
+	if [[ "$NIMBLE_DIR" != "dist/nimble" ]]; then
+		mkdir -p dist
+		rm -rf dist/nimble
+		ln -s ../"$NIMBLE_DIR" dist/nimble
+	fi
+
+	# bootstrap the Nim compiler and build the tools
+	rm -rf bin/nim_csources
+	cd csources
+	if [[ "$ON_WINDOWS" == "0" ]]; then
+		$MAKE clean
+		$MAKE LD=$CC
+	else
+		$MAKE myos=windows $UCPU clean
+		$MAKE myos=windows $UCPU CC=gcc LD=gcc
+	fi
+	cd - >/dev/null
+	if [[ -e csources/bin ]]; then
+		cp -a csources/bin/nim bin/nim
+		cp -a csources/bin/nim bin/nim_csources
+		rm -rf csources/bin
+	else
+		cp -a bin/nim bin/nim_csources
+	fi
+	sed 's/koch tools/koch --stable tools/' build_all.sh > build_all_custom.sh
+	sh build_all_custom.sh
+	rm build_all_custom.sh
+
+	# update the CI cache
+	cd - >/dev/null # we were in $NIM_DIR
+	if [[ -n "$CI_CACHE" ]]; then
+		rm -rf "$CI_CACHE"
+		mkdir "$CI_CACHE"
+		cp -a "$NIM_DIR"/bin/* "$CI_CACHE"/
+	fi
+}
+
+if nim_needs_rebuilding; then
+	build_nim
+fi
+
diff --git a/nim_needs_rebuilding.sh b/nim_needs_rebuilding.sh
deleted file mode 100755
index a1553597b..000000000
--- a/nim_needs_rebuilding.sh
+++ /dev/null
@@ -1,13 +0,0 @@
-#!/bin/bash
-
-# this one is required
-set -e
-
-# script arguments
-[[ $# -ne 2 ]] && { echo "usage: $0 nim_dir nim_binary"; exit 1; }
-NIM_DIR="$1"
-NIM_BINARY="$2"
-
-# compare binary mtime to the date of the last commit
-! [[ -e $NIM_BINARY && $(stat -c%Y $NIM_BINARY) -gt $(cd "$NIM_DIR"; git log --pretty=format:%cd -n 1 --date=unix) ]]
-