I have been looking into `make clean-images images` performance, and realized jmod keeps compressing files with default compression level. Tuning that toward lighter compression levels improves build performance considerably, without a heavy loss in *.jmod sizes.

Example after JDK-8293532:

# ----- Baseline

real 0m10.346s
user 1m5.007s
sys 0m10.424s

81476 build/linux-x86_64-server-release/images/jdk/jmods/

# ----- Patched, level 0 (good for interim JMODs)
real 0m6.419s
user 0m57.435s
sys 0m10.991s

184008 build/linux-x86_64-server-release/images/jdk/jmods/

# ----- Patched, level 1 (good for final JMODs)
real 0m9.237s
user 1m0.818s
sys 0m10.530s

85760 build/linux-x86_64-server-release/images/jdk/jmods/