I have this problem too. I created a project in UE 4.26 with excellent performance. I upgraded to 5.4 and the performance got worse (the game doesn’t go over 20 fps, although sometimes it flickers and reaches 60fps for a frame or 2).
Over time, 5.5 came out and I started another mobile game there, that game has decent performance, not WOW like in 4.26 but decent overall, so I decided to migrate the project from UE 5.4 to 5.5 and the performance is still horrible. It feels to me as if the project is bugged or unable to update.