I have had YouTube since the latest Ultra and I felt 1 FPS that has been since my mobile fell on the floor but all other apps and games run smoothly like never before because of an update (Huawei P samrt Z 2019)
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The memory may be overloaded by many tabs or open apps. However, it can also be due to an overflowing main memory, because YouTube also creates cache files. However, it can also be due to outdated software, you can try to update it or simply restore your system. But it can also be due to YouTube itself and you have to wait for the next app update.
But it may also be that you have set the video quality too high and your device processor can't handle it, but it is extremely unlikely
Try to reinstall YT or you just land YouTube on the internet
I've already