The worst experience for me is trying to play videos in the google photos mobile app (or the website). It can take 1-2-3-4 minutes (or never) for it to finally play a video I uploaded from my phone a few days, weeks ago.
My god the google photos mobile app is oblivious, if I select a photo and make the mistake of swiping to the previous or next one, is game over for me, only a black screen awaits my despair. I have to close and reopen the app (same for web version) and then, only if I select the video/photo correctly the first time, after a lot of waiting, I can finally view my content.