"jruby -e 1", cold start, just took 4 seconds for me. I use jruby interactively almost every day (it's extremely handy in penetration testing), and I'm pretty sure jruby is noticeably slower to start than MRI.
It's definitely noticably slower, but 4 seconds per startup would make it unusable for me. However, since it's
time jruby -e 1
real 0m0.514s
user 0m0.510s
sys 0m0.040s
I find it acceptable (could it be the SSD in my laptop that makes such a large difference?). As http://jira.codehaus.org/browse/JRUBY-5181 shows, as soon as some gems come into the picture, actual startup times deteriorate rapidly.