1.13 changes the format in which the world is saved slightly. Chunks that are stored in the old format are automatically converted when they are loaded, but this conversion takes a bit of extra time, so expect loading chunks for the first time in 1.13 to be a bit slow. If it takes you a long time to log in for the first time, or when you're using portals to somewhere you haven't been in 1.13, then you just need to be patient. If you see a chunk that hasn't loaded, you also just need to be patient. Whatever you do, don't walk into unloaded chunks, because then you might fall to the void.
The view distance has been decreased temporarily, it will remain low for a while after the reset, and then increased again in time.
We've got a program that will upgrade chunks in the background, so that eventually every chunk will be upgraded to the new format. However we expect this process to take several months. So everybody will just have to live with the extra lag until that is finished, but rest assured that all chunks will be upgraded, so the extra lag is only temporary.