Occasional when I go through a nether or end portal to many times within a short amount of time, especially in creative mode, the loading screen just says building dimension for an excessively long amount of time. When this happens I normally just restart the application because it seems to take forever. It does sometimes finish loading, but most of the time it takes over a minute and I just restart the app. I have only experienced this problem in a survival world with an add-on while re-spawning from the end and returning to the end, and in a creative world while linking up nether portals since I was using the nether portal a bunch.