That's crazy and there's definitely something going on! The 99% cpu warning from AC is one thing, but it only means that the cpu took too long for the physics to complete in time, NOT full CPU load overall.
However this shows full load across all cpu cores, and AC can't use more than 3-4 cores, so there's something going on.
Sadly there's no way to easily track the cpu load of applications. The easiest way is to look at the Taskmanager on a second monitor, but you need another monitor for that..
Another way is to record the Taskmanager window with OBS, while AC is in the foreground.
But depending on your hardware, that might cause even more load on cpu and gpu
One solution would be to switch AC into Borderless mode (alt+enter or not select Fullscreen in content manager) and then have the Taskmanager in the foreground.
AC loses a bit of performance that way and gsync/freesync and vsync might disengage so you'll have tearing and stuttering.
BUT there's definitely something really really wrong, so for a rough check, this might be the way to go.
If Taskmanager only shows AC with a high cpu load, then it's time to use the CSP render stats app. You need the CSP extensions: GUI and I think "Taskbar" to enable that one.
It shows every single thing AC is doing. CPU and GPU time.
Yep
Hehe no worries! I hope you can solve it! If not, I'm happy to do a little Teamviewer Session or so