Update your post to include HP Omen 15 specs and OS information.
Use Task Manager and Resource Monitor to observe system performance before gaming, while gaming at 60 fps, and then when at 20 fps.
Use both tools but only one tool at a time.
Open the tool, leave the window open but drag to one side or the other so you can continually watch. (A second monitor would be very helpful if at all possible.)
Learn what all is running in the background. Could be some app is launching and causing the fps to drop.
Objective is to discover what changes when the fps drops.
There is no need to rush through it all. In fact, you may need to work through it all a few times just to become comfortable with both the troubleshooting process and each of the tools.
Another tool that can help is Process Explorer (Microsoft, free).
https://docs.microsoft.com/en-us/sysinternals/downloads/process-explorer
See what else you can discover - good chance that there are other clues to be found.