I would not say that the C# runtime is slow, but it is a JIT compiler that is not optimizing for start-up (as far as I know), and it is "doing a lot of work" at runtime to achieve the eventual performance it is capable of. Start-up time is not the most important property for a lot of services, but for user applications it's pretty high up there, so if the underlying runtime is not optimizing for that this shows a disconnect in the choice of technology stack.

I'm quite impressed of both .NET and OpenJDK in some metrics, but it is often not resource efficient, which is something I do value.

One example of an application that works as I would expect others to do is MuPDF, Being able to open 20MB+ PDFs in 1/10 seconds on a 10 years+ old laptop.

By the way, does anyone know why Debian launches LibreOffice so much quicker than Ubuntu, Fedora or Archlinux (or any other distro I've tested with)? In Debian its 1-2 seconds, and the others 5-10 seconds. I mean it could be included extensions or how they are configured, but I'm honestly interested.