EMUI 9 always ran crap on my mate 20 lite. Even after factory reset and clearing cache made no difference. With all closed I had 1.70gb free ram. Open the camera and you really started noticing the lag and stuttering, switching between camera and gallery ram dropped to 300mb free. Then we had battery life, 10-20% more drain.
After a factory reset and attaching to computer I saw the option restore to previous version, so went back to 8.2.0. Free ram is 2.20GB, with camera open and switching about ram 1.5gb, battery drain gone.
Something is wrong with 9.0 it has ram issues and battery drain.