Well, you posted just in time for a weekend. Maybe people were away.
It also seems like an unclear question. If the game takes 100% CPU when loading, that's fine. It's just your computer trying to do things as quickly as it can. If you're saying the game always takes 100% CPU time for one core, then that is not typical. Do you use an antivirus that does real-time checking of running programs? That could explain it. If you're asking why UnderRail doesn't use all your cores, that's easy - balanced multithreading is hard and can't really be justified for a game like UnderRail (and a dev team as small as StygSoft).
One core running at 100% is no threat to your computer safety. Should be fine, no matter the cause.
