When playing split screen on our Xbox One X the game starts severe lagging (e.g. most severe when picking items in the inventory, a little less when placing blocks, but also very noticeable) as soon as a third player joins in splitscreen. Interestingly, if one of the three players moves into the nether (via a portal) the lag is gone for all players, as long as the player stays in the nether. If the player returns to the normal world the lag returns immediately. Note: playing in split screen for only two players works fine.