Jasper is *extremely* slow at compiling .tag files packaged in a .jar -- tens of seconds per JSP. One cause is that the javac compiles are very slow, despite there being relatively little java code to compile. (About 60% of total time.) This is probably because the compiler is being invoked once for each .tag, rather than just once for all the generated .java files. If they could be batched up and compiled at the end I suspect there would be a huge improvement. (All of Tomcat/Jasper takes less time to javac on my machine than running up one .jsp that includes a lot of .tags.)
I've no doubt this could be better. The fix for bug43878 may offer some improvements when not using development mode. Improving this further is likely to require fairly major changes to Jasper so I am marking this as an enhancement. If you (or anyone else) feels the urge to scratch this itch then patches (in diff -u format) are always welcome.
No further interest in this. Pre-compilation is the obvious solution here.