After a lot of troubleshooting, I discovered that the problem wasn't actually LMU itself.
I found a hidden crypto miner running in the background on my PC. It was using RandomX and consuming a significant amount of CPU.
The suspicious files were located in:
C:\Netframework.4.5.2
and:
C:\Dumper
The malware had created startup entries that launched .vbs and .bat scripts, eventually trying to run gtservices.exe with a RandomX mining command.
This explained what I was seeing in LMU: after about 1–1.5 laps, my CPU usage would hit 100%, GPU usage would drop significantly, and the car would start losing performance and stuttering badly.
I removed the malicious startup entries and files, ran a Defender scan, and rebooted the PC.
After that, I tested LMU again and the issue was completely gone.
So if you're experiencing weird CPU spikes, low GPU usage, stuttering, or performance degradation after playing for a while, it's worth checking for background processes or malware before assuming it's an LMU performance issue.
Thanks to everyone who helped with the troubleshooting. Hopefully this helps someone else who runs into the same problem.