Aye, 64-bit hotspot with implicit -server switch, but going on your astonishment I took a look and it seems the 0.5s time is only achieved with fsc running from previous runs of the REPL. So from cold, i.e. no fsc running it takes about 3 seconds. 2.9.0-1 was taking 7-8 seconds to get the REPL prompt regardless of whether fsc was running already, so this is still like night and day for me :-)

Amongst other things it looks like Paul Phillips (a.k.a extempore) has done a lot of optimisation work. This looks like a pretty important one:

https://codereview.scala-lang.org/fisheye/changelog/scala-sv...