Startup times has definitely improved for recent versions, but it is still bad enough that i do not use jruby for normal development. There may be no easy way to fix this issue, but I at least want to report it so that the issue is known. Here are some sample startup times on my intel core 2 duo machine Ubuntu machine.
ruby --version: ~.5s
rake --version: ~1.5s
buildr --version: ~2.75s
The trend can be explained by the fact that rake runs on top of jruby and buildr in turn relies on rake. This seems to show that most of the .5 seconds startup time for ruby is not constant and does not armortize out for larger programs. Instead, the larger programs load many times more slowly.
Ruby's startup time has definitely improved from the time I first submitted this issue, but once one begins to run ruby programs of moderate size the startup time becomes unbearable. 3 seconds is too long to wait just for buildr to fire up, let alone do anything, especially when the task it will execute may take less than a second. The same issue applies to rake. Another example of the impact of this startup time is that if one has a syntax error in his or her build file, each edit/compile cycle will be at least 3 seconds. With MRI 1.8.7, buildr starts up in ~.275 seconds, or in 1/100 the time.