Rails asset precompile times on JRuby are considerably slower compared to MRI. I came across this post which provided suggestions on speeding up the asset precompile task.
Using the following options – using Node.js instead of therubyrhino for JS compilation, forcing the JVM to 32 bit (although this can be omitted on a 32 bit JVM) and not using JIT compilation – cut my asset precompile time from 4 mins 37 secs to 2 mins 8 secs. Using Node.js contributed to the majority of that time since I’m using a 32 bit VM.
EXECJS_RUNTIME='Node' JRUBY_OPTS="-J-d32 -X-C" rake assets:precompile