Ah, yes this seems like the same issue I posted about here: Compilation taking twice as long in UE5: Number of processes being limited?
Check the above post for more info and details regarding this. It seems like they might have changed something regarding to how many processes are used (or some other factor that affects overall CPU usage) when compiling in UE5.
The only workaround at the moment for some people seems to be the above BuildConfiguration xml change, although it doesn’t lead to any improvement on my environment.
I think it’s up to Epic to fix this one, but as there hasn’t been an official response yet, the current status of this issue is unclear.
It majorly affects productivity, so fingers crossed.