diff --git a/build_image b/build_image
index 2a8dc2d75c..971f0a1555 100755
--- a/build_image
+++ b/build_image
@@ -59,8 +59,8 @@ DEFINE_integer statefulfs_size 1024 \
 DEFINE_boolean preserve ${FLAGS_FALSE} \
   "Attempt to preserve the previous build image if one can be found (unstable, \
 kernel/firmware not updated)"
-DEFINE_boolean fast ${FLAGS_FALSE} \
-  "Call many emerges in parallel (unstable)"
+DEFINE_boolean fast ${DEFAULT_FAST} \
+  "Call many emerges in parallel"
 
 DEFINE_string usb_disk /dev/sdb3 \
   "Path syslinux should use to do a usb boot. Default: /dev/sdb3"
diff --git a/common.sh b/common.sh
index a6957fad77..5258c1a6f8 100644
--- a/common.sh
+++ b/common.sh
@@ -105,6 +105,12 @@ ALL_BOARDS=$(echo $ALL_BOARDS)
 #DEFAULT_BOARD=x86-generic # or...
 DEFAULT_BOARD=$(echo $ALL_BOARDS | awk '{print $NF}')
 
+# Enable --fast by default on non-official builds
+DEFAULT_FAST="${FLAGS_TRUE}"
+if [ "${CHROMEOS_OFFICIAL:-0}" = "1" ]; then
+  DEFAULT_FAST="${FLAGS_FALSE}"
+fi
+
 # Detect whether we're inside a chroot or not
 if [ -e /etc/debian_chroot ]
 then
diff --git a/mod_image_for_test.sh b/mod_image_for_test.sh
index 52712822b7..bc6a2023b8 100755
--- a/mod_image_for_test.sh
+++ b/mod_image_for_test.sh
@@ -32,7 +32,7 @@ DEFINE_string qualdb "" "Location of qualified component file" d
 DEFINE_boolean yes $FLAGS_FALSE "Answer yes to all prompts" y
 DEFINE_string build_root "/build" \
     "The root location for board sysroots."
-DEFINE_boolean fast ${FLAGS_FALSE} "Call many emerges in parallel"
+DEFINE_boolean fast ${DEFAULT_FAST} "Call many emerges in parallel"
 
 
 # Parse command line