application performance bad on ios device but good in simulator

I use opengl with QOpenGLWidget， render yuv data to play video. In the simulator，nothing is bad。when I run it on my iphone device, it is strange to perform fluently when there is only one window but bad in mutile windows。after I check my code，in the function painterGL()，if I disable glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT )，then it work fluently in mutile windows but I have no picture to see。As I am not familiar to quick qml，so I develop with qwidgets。