why does Terminal run about half speed when sleeping

I frequently run simulations from Terminal that take many hours, often overnight. Normally I use the Caffeine menu-bar app to prevent sleeping during these runs. If I forget to do that, **the job continues to run during sleep, but at about 50% of normal speed**. Does anyone know why that happens? My first guess was a power-saving strategy, but my laptop power adaptor is always plugged in. This is not a critical problem for me, I'm just curious. Normally I disable sleeping, and if I forget, at worst I get about half as much work done overnight than I expect.