I have this problem, it is very bad, I have a 1 1/2-year-old long term survival world that I decided to turn into a realm because all of my family and some of my friends play on it, (this world is 360+ Mb big IDK if this has anything to do with it) But I am regretting converting it because the blocks lag soooooo much, I have to break a block 3+ times before it actually breaks, and it sometimes takes 10 seconds to eat one piece of food, and the inventory and chests open 5 seconds after I push the button, sometimes block that get broken, delete from existence. This only happens though when there are two or more players on the realm.
(you can see in the video I have attached; "realm lag.mp4)
nevermind this was fixed after restarting my nintendo switch