Exactly, this is not jruby being slow but a particular ruby program being slow on jruby. There is a reason for that and it is extremely likely to be a fixable problem. This is code that was never optimized for the JVM so there is probably all sorts of stuff happening that makes a lot of sense on MRI that is maybe a bit sub optimal on the JVM.

One thing that comes to mind is that a lot of performance critical stuff in ruby is implemented via native libraries. The Jruby ecosystem has alternate implementations for a lot of that stuff. But it is probably also able to interface with native code directly. That sounds like that might be a little bit of a bottleneck potentially. And any alternate java based replacements for whatever is being called might have its own issues/bugs/etc.

But instead of hypothesizing what the problem might be (and getting that wrong repeatedly), profiling tends to be much more effective indeed. I've done this a couple of times to diagnose performance issues and it rarely is anything you'd expect. Once you know where it is spending its time, you can usually mitigate the issues. Use a profiler, add some logging, instrument the jvm, etc. There are lots of ways to do this. Even just knowing how often it starts a new process would be good to know. It's apparently more than once because otherwise you'd expect --dev to not speed things up like it did.