Sometimes, when you get near the ruins at the bottom of the ocean [where there's drowned(s)] your game will significantly lag to about 1fps (and i have an RTX 2070 and I7-9700k so it's not hardware that's the issue), it makes the game unplayable when you're near the ocean, but doesn't bother you if you unload the chunks, and it will only happen in some ruins, i suspect that a glitched chest is to blame here, as i once found one which couldn't be opened, and it got laggy near it, but i have no proof of this, so it might be something else.
If you refresh the chunks the lag will stop for a second, further proving that loaded chunks are to blame, but i have no idea on how to stop it, as i can't really do anything in the area without spending like a full day to do an hour worth of work.