I'm having a similar problem. My game runs very well with APK signed on my old phone (android 7), but when testing on new phones (android 9/10), some just run extremely slow.
It is very strange, because in random cell phones they run well, and others extremely slow.
Is there any way to debug on these phones to identify the problem?
The APK was built by C3... Testing by remote preview, on all phones ran perfectly