Having the exact same problem on 2017 Ipad, Since the 1.5.0 update, My kid is going crazy minecraft is his life.
Very slow to load, Wont load worlds - returns to world select screen like yours, Can i ask, when you go to settings and you go to storage, manage storage and try to browse your worlds does it hang there also?
Have closed ticket and recreated as MCPE-35827 - Worlds intermittently not loading