Playback lagging after alt-tab.

My playback working smooth and perfect until i open another window(i do alt tab to another app). If i open new window of any app/program and then going back to Premier Pro i need to wait for 5-15 sec until playback start working. Sometimes sounds starts working but not a video and i need to wait additional 5-10 sec to see a video. That is happening even if you do alt tab for a few seconds. 
I tried to prerender video, cache clean, GPU processing etc. nothing working. 
How can i fix it in other way?