Perhaps the original Windows 11 upgrade was buggy or corrupted in some manner.
Next time you experience a performance drop look in Task Manager, Resource Monitor, or Process Explorer.
Process Explorer may need to be downloaded - Microsoft, free.
https://docs.microsoft.com/en-us/sysinternals/downloads/process-explorer
Use the tools (one at a time) to determine what system resources are being used, to what extent ( % ), and what is using any given resource.
First look now while everything is snappy and fast. Print out some screenshots for reference/comparison purposes later on if necessary.
Take the screenshots right after initial boot when the system is stable, then a set while doing light work or browsing. Then a set while gaming.
Then, if the problem returns, you can compare the "now" to what was working before. May provide a clue or two.