That's odd...are you running potplayer off of a hard drive or ssd? Which OS are you using? Are you using the 32 or 64 bit version of potplayer? I'm running it off of an ssd and using Windows 10 x64 1607 and using the 64 bit version of potplayer. I tried playing some 4k videos and they started instantly and played very smooth. Not sure what's causing it for other people.