What I agree on is that if we had modern .NET available we'd get a free 2-3x improvement, it would definitely be great. BUT having said that, if you're into performance but unwilling to use the tools available then that's on you.

From the article it seems that you're using some form of threading to create things, but you don't really specify which and/or how.

The default C# implementations are usually quite poor performance wise, so if you used for example the default thread pool I can definitively say that I've achieved a 3x speedup over that by using my own thread pool implementation which would yield about the same 30s -> 12s reduction.

Burst threading/scheduling in general is also a lot better than the standard one, in general if I feed it a logic heavy method (so no vectorization) then I can beat it by a bit, but not close to the 3x of the normal thread pool.

But then if your generation is number heavy (vs logic) then having used Burst you could probably drop that calculation time down to 2-3 seconds (in the same as if you used Vector<256> numerics).

Finally you touch on GC, that's definitely a problem. The Mono variant has been upgraded by them over time, but C# remains C# which was never meant for gaming. Even if we had access to the modern one there would still be issues with it. As with all the other C# libraries etc., they never considered gaming a target where what we want is extremely fast access/latency with no hiccups. C# in the business world doesn't really care if it loses 16ms (or 160ms) here and there due to garbage, it's usually not a problem there.

Coding in Unity means having to go over every instance of allocation outside of startup and eliminating them, you mention API's that still need to allocate which I've never run into myself. Again modern isn't going to simply make those go away.

Regarding GC pauses, there is an interesting alternative GC with ultra low pauses for .NET called Satori. It's primarly discussed here https://github.com/dotnet/runtime/discussions/115627, and the GC itself can be found here https://github.com/VSadov/Satori