It's just midi loops. Well, some instruments are loaded with effect chains (both midi and audio) and one of them is a vst (but removing it does not change much).
Actually I tried running the same project again and this time it worked fine (CPU load indicator always below 80%, no problems with outgoing audio). Can't explain why since I had the same exact routine tasks open in the background and both times the laptop was plugged in. Anyway: 80% of CPU load is still concerning and I'd like to know whether an external sound card would help reducing CPU stress. As I wrote in the previous post I'd buy one anyways since I need inputs and outputs.
Is there a particular kind of sound card that would help solving the CPU load problem?