Problem details:
So first, I had a 100% GPU usage issue that arised ALWAYS and the WHOLE time I played osu (it happens both during playing and in the main menu etc. etc.). There were also some lags that someties appeared out of nowhere.
I first thought it had to do something with not being able to run external fullscreen but the fullscreen error message disappeared
This also happens in both the latest stable version and osu lazer.

I was able to fix it by enabling compatibility mode which made the gpu usage go down to about 10-20%.
This was obviously not a permanent fix since it made alt-tab not properly working anymore, created big stuttering for a few seconds when alt-tabbing back to the game, sometimes mouse issues where happening when alt-tabbing back to the game like clicks not being registered at all (also in the main menu) or both the windows and the in-game mouse overlapping each other.

I tried uninstalling the Nvidia GeForce Graphics Driver that was installed on my laptop. This kinda helped:
gpu usage went down to 40-60% (which is still much to high) without compatibility mode turned on. There were also random 100% gpu usage jumps during playing + main menu and random 0.5 second stutters.

Turning on "reduce dropped frames" didn't help at all.
Running compatibility mode after this "partly fix", gpu usage still goes down to about 10-15% and the lagging still disappears.

Video or screenshot showing the problem:
Compatiblity Mode turned on: https://drive.google.com/file/d/1zIax1BtBijhTKp7KI1FxdNFp3r27hq6O/view?usp=sharing
Compatiblity Mode turned off: https://drive.google.com/file/d/1Ih98PO6De07qZpzoY3YIqnQ3SdhxFrm2/view?usp=sharing
https://drive.google.com/file/d/12IWltVy-hTVsH5aglNg7hTKrnRdfYCE1/view?usp=sharing
=> you can see the lag spikes in the gpu history graphs in the bottom left corner
Osu Lazer Fullscreen Error: https://drive.google.com/file/d/1uIomSyFs4VHrsJYvpdyWJq-meqw2yBYO/view?usp=sharing (same error message in osu before latest stable version)

Stats:
- Windows 11 64-bit Predator Helios 300
- GeForce GTX 1660 Ti
- Intel(R) Core(TM) i7-10750H CPU @ 2.60 GHz
- 16GB RAM

Also helpful to know:
- the cpu and memory usage is always normal no matter if I have compatibility mode turned on or off, as well as my integrated gpu not being used by osu
- osu and Windows installed on SSD (200+ GB free space left)
- I'm always using the "VSync" frame limit option
- the external fullscreen still doesn't work in osu lazer at all

I hope this is enough info, thanks for reading through!

osu! version: Stable 20221016.2 (latest)

Are you using the correct GPU? Kinda sounds like youre using your igpu by mistake somehow, cause its incredibly hard to fully use a GeForce GTX 1660 Ti with just osu!. Have a check in nvidia control panel to make sure youre using your dedicated gpu for osu! and not the integrated one and switch if needed.

If its not that, does this still occur with the default skin enabled?

thanks for the reply.
Yeah I've checked nvidia control panel and I did select "max performance mode" for battery mode and for gpu I used my GeForce GTX 1660 Ti. The gpu issue still exists without compatibility mode turned on even when I use the default skin.
I've also selected high performance for osu in the windows graphics settings.
osu default skin: https://drive.google.com/file/d/1Kn1ozOttooUfOqnZUTQeN62lLcPlCZQ1/view?usp=sharing
nvidia control panel: https://drive.google.com/file/d/106QJgZ80Hv9X7ywCRctwfoFX3c-Dt-YD/view?usp=sharing
But yeah, I'm not totally if osu actually uses the gtx gpu since when I check the task manager, it shows usage in both integrated and gtx (I of course have any other gpu related program and games closed while using osu)

Alright, small notice:
I disabled my integrated graphics card but osu won't use my gtx graphics card even then but instead use the windows basic graphics adapter instead that's automatically installed when disabling integrated graphics card. Osu is also giving me warning that I should update my graphics driver and that it was forced to run in compatibility mode. So I guess osu isn't fully using my gtx graphics card but at least partly also uses the integrated one (I don't know if that's normal for apps, but might be worth mentioning anyhow).

Alright, I also just reinstalled osu. The first time I started osu after reinstalling, it also gave me a warning saying that the compatibility mode has been forced for better nvidia optimus or something. I was able to disable compatibility mode and run it without though that made the issues appear again.