Recently discovered that running with -XX:TieredStopAtLevel=1 can speed up jmod execution a bit:

http://mail.openjdk.java.net/pipermail/jigsaw-dev/2016-May/007593.html

Some experimental results on my machine:

time make images

real 2m39.281s
user 42m15.164s
sys 3m56.388s

-XX:TieredStopAtLevel=1 for jmod target only:

real 2m34.913s
user 38m18.112s
sys 3m44.528s

-XX:TieredStopAtLevel=1 for all small java targets in makefiles:

real 2m29.490s
user 35m45.944s
sys 3m33.428s

Reports indicate the real time improvement is even better on smaller machines.