Followup from before: If you have seen my previous post, since I fully reinstalled the game (on both my wife's and my own Switch), we have not had this problem at all. I would strongly recommend that anyone with this issue trying a reinstall and seeing if that fixes it.
To note, you do not have to delete any save data, just the game itself. (If you have a save file causing a problem, it's a different bug.)
So, smallish update for my experience. I tried uninstalling the game and then did a fresh install (left the save files alone) and haven't had the issues anymore. I tried that after my friend had installed it for the first time and he had no issues playing, online with us or not.
No guarantee that it is actually fixed, but now I wonder if it could be that the issue is a piece of code that was remaining through updates and so needed a clean install of a more recent version (which had assumedly fully removed that code) to get rid of the problem.
Would appreciate if anyone can see if this is not just me being lucky (or lucky so far), since if this is the case then a clean install with the newest release would solve it.
@Eli: The game failing to load textures for sections can occur when there are memory issues (or just loading a lot at once), so it could certainly be related, though in your case it isn't likely more than a tell that a crash is about to happen. (It is normally not a big deal when textures don't load immediately and the game will just finish loading the textures late.)
My wife and I are currently having the same problem, limited to the Switch release. We ended up starting a Realm just so we could move her world to the XboxOne for her to play on.
The problem is two-fold: first, everything becomes laggy (with a small chance of recovering) and second, when this happens often the only fix is to return to the Home menu and close the game, which will take much longer than it should (and if you close it when the problem is not occurring the game will close quickly)
We've noticed that there are several "triggers":
-Starting up: When you load in on Switch, the game first goes through a couple of checks to make sure you are logged into your Microsoft account and that everything is synced properly. During this step, trying to do anything can cause the game to do everything extra laggy, though if you wait a little bit it should finish the process ok. HOWEVER, if you stay in the main menu for very long (like, a couple minutes?) then it will hit the perma-lag that you then have to close the game.
-Accessing the Marketplace or Profile seems to easily lock in the perma-lag unless you time accessing them just right after the game hits that "safe" time.
-While playing, at any random time, the game may start lagging; you can move around like normal, but breaking blocks, creatures moving, etc. will not be updated (wonderful when a currently invisible Creeper comes along...). The loading image will pop onscreen every time you try to open a chest or your item menu as well. If you are lucky, it will recover and let you continue playing. Otherwise, Home and quit.
Also, this seems to be very likely to happen within 10-30 minutes of starting to play in multiplayer (online/network) worlds. Haven't tested with split-screen.
It seems like the problem is a weird memory issue, just based on how long it takes to close the game when this happens...?
@Herbubulle: That does not sound like this bug (when you say "feezing a lot", do you mean a full crash or just laggy? If it's just laggy, then it definitely isn't this bug. Also, this hasn't been noted before to be triggered by skin swapping, though I've noticed the game lag a bit sometimes if skin swapping happens during split-screen play). It doesn't help to report different bugs to a different report, each different bug needs to be reported separately to be handled correctly.