when a player joins a lan world, the world starts lagging for the player and the host, not meaning of the game starts slowing down, but entities kinda do
the entities starts moving way slower than they usually do, during raids is a bit annoying because you kill the pillagers before they enter the village
the other players speed is completely fine, but when a player is flying, it kinda does a "lag spike", because they stop midair or pushes it back
Please check if that's still an issue for you in the latest 1.14.4 (pre-)release.