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.

Maybe you're using the older Java based libreoffice on the other machines? It was mostly rewritten in C++.

That's pretty odd to assume. All but Debian are running more or less the latest, and Arch is on the "fresh" track.

Did not assume. Asked, maybe the other distros are old versions. Not sure why else there would be a huge difference... maybe flatpack/snap dependencies not in fs page cache.

Ok, sorry for assuming your intent. No, its nothing like that. They are all the distro provided stable versions installed as "regular" applications.

And it seem to be the start up process that differs, as putting them all in a ram-disk does not alleviate the issue, and restarting the app cuts the time in ~1/2 but equally for each distro.

My guess, as I said first is what default libraries as loaded, and possibly how they are configured. I do however find it strange that this has not been mentioned elsewhere as I've been struck bu this difference for years, when I happen to load a pure Debian install (not what I usually use).