@Tex99 I'm currently considering upgrading from a OnePlus 6 and found this thread.
May I ask if you have tried to force 90 Hz via adb (otherwise it will auto-switch between 60 and 90 Hz) and if you can experience the same issue with the UFO test in a browser:
https://www.testufo.com/
I've had a similar issue on the OnePlus 6 at first but I think after OOS 10.3.0 it was fixed. Scrolling was very laggy and touch latency was beyond acceptable. They seemed to have sorted that out for me (luckily).
EDIT: As far as I can tell the issue is also present on my OP6 (haven't noticed it though). You can see how at least 2 frames are dropped in this video:
Back on the OnePlus 5T I had an issue with the audio playback. The reactions to this were quite similar. Some people started to notice it after I told them, others never heared it. But it was present on 2 devices with every ROM, kernel or firmware:
https://forum.xda-developers.com/oneplus-5t/help/audio-issues-related-to-fingerprint-t3860937
OnePlus never responded and even with my connections and contacts back then, I never could get them to fix it.