I get the feeling that there has been a lot of low-hanging optimization fruit in .NET that went unaddressed because a) the original compiler was too complicated to change and b) there was no pressure to improve it because it only ran on Windows and there were no real competitors.

a) was addressed by Roslyn, which is apparently much more manageable, and now that .NET is supposed to run everywhere, it has to have competitive performance. Hence the big gains.