You not alone there, flown yesterday night from Hannover to Kraków, 7800x3d/4080 Super and cpu was getting work out, strange thing once near Poland everything went back to normal. CPU utilisation was anything from 30-75% over germany, temps spikes, luckily I don't suffer stutters, UK is the same for me and US for example is smooth sailing. I have no idea what they done in that sim and what is causing the problem. I don't get this issue with other sim. While back I posted on msfs forum so did other users but no one found a solution. Both times I noticed this was using Fenix A320. It can't be related to settings as I would get this everwhere.