--- a/client.mk
+++ b/client.mk
@@ -103,16 +103,24 @@ define CR
 
 endef
 
 # As $(shell) doesn't preserve newlines, use sed to replace them with an
 # unlikely sequence (||), which is then replaced back to newlines by make
 # before evaluation.
 $(eval $(subst ||,$(CR),$(shell _PYMAKE=$(.PYMAKE) $(TOPSRCDIR)/$(MOZCONFIG_LOADER) $(TOPSRCDIR) 2> $(TOPSRCDIR)/.mozconfig.out | sed 's/$$/||/')))
 
+
+# Automatically add -jN to make flags if not defined. N defaults to number of cores.
+ifeq (,$(findstring -j,$(MOZ_MAKE_FLAGS)))
+  cores=$(shell $(PYTHON) -c 'import multiprocessing; print(multiprocessing.cpu_count())')
+  MOZ_MAKE_FLAGS += -j$(cores)
+endif
+
+
 ifndef MOZ_OBJDIR
   MOZ_OBJDIR = obj-$(CONFIG_GUESS)
 else
 # On Windows Pymake builds check MOZ_OBJDIR doesn't start with "/"
   ifneq (,$(findstring mingw,$(CONFIG_GUESS)))
   ifeq (1_a,$(.PYMAKE)_$(firstword a$(subst /, ,$(MOZ_OBJDIR))))
   $(error For Windows Pymake builds, MOZ_OBJDIR must be a Windows [and not MSYS] style path.)
   endif