I don't really consider it to be slow at all. It seems about as performant as any other language this complexity, and it's far faster than the 15 minute C++ and Scala build times I'd place in the same category.
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In C/C++ I don't even have to worry about it.
Out of interest what were they?
The one that immediately comes to mind is cvc5... not super recently though.
I suspect that "tried to" is doing a bit of work here. The fact that it was failing and swapping out probably meant that the more memory heavy g++ processes were going slower than the memory light ones, resulting in more of them running simultaneously than would likely have happened in a normal successful build. Still, this was on a system with 32GB of ram, so it was using roughly that before swapping would slow down more memory intensive processes.