Low framerate

I am on a new survival map in the Minecraft: Better Together version on Xbox One. On a certain part of my map (close to the main house), whenever I or anyone else walks in that area, the framerate tanks (3-10fps). As soon as we walk out the area, the framerate increases. The affected areas are a few blocks in diameter, and the low-framerate zones move from time to time.

I have dug around in those spots and there is nothing out of the ordinary going on. Just stone and empty caves.

Extremely low framerate also occurs when a zombie spawner is on and has spawned several mobs. But even when the spawner is off and there is no abundance of mobs present, the game will run at a shaky 20-30fps for no reason.

It's a week-old map with just a few simple structures that's offline only. What is going on?