In practice I've found the start-up times aren't all that different. The drag is more pronounced on older hardware, though. A current i5 or i7 system with an SSD is usually roughly the same.
That is very contrary to my experience. A Rake task in a Rails project almost always takes somewhere like 15-20 seconds just to start up (on MRI it's around 2-5). Just today, I tried to run the Middleman static site generator. Where on MRI, Middleman starts building after around 3 seconds, on JRuby it starts building after around 10 seconds. And I'm on a 2012 Macbook Air.
