[Bug report] system start to be slowen down after playback

Hi, I am suffering a quite strange problem.
I am using Nexus 7 LTE (2013), mx player was always working quite well until I OTA to Android version LMY48P about 1 month ago...
Whenever I start to playback, the android system UI starts to be slowen down, which means the FPS of system UI has been decreased and it looks like slides...
No matter I pause the video, I stop the video, I quit mxplayer or I force stop mxplayer, the system UI wonldn't be normal unless I reboot my Nexus 7.
I was afraid that this might be due to OTA, so I wipe my Nexus 7 clean and reinstall the system using img downloaded from Google developer site, and the bug didn't go away...
I also tried to use s/w or H/W orH/W+, they cause same problem...
It is strange that if I only open mxplayer, nothing wrong. But whenever I pay something, bug occured...
I also tested that it is only the system UI is slowing down but loading and performing of apps are not infectes...
Any ideas guys? It's just me or a common problem?

Android version 5.1.1 LMY48P, mxplayer version 1.7.40.