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.
Yes, this is an excellent point I forgot. With closed-source, you only get to make perf improvements if your manager decides there's not a more important feature you should be working on instead.
For example, .NET doesn't do escape analysis, though I've read this is less of an issue for .NET than Java because .NET has struct value types instead of everything being a heap-allocated object.
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.