If you make changes to a tilemap prefab by placing tiles and then apply the overrides to the base prefab, applying the changes is quite slow and can take a long time. However it only seems to be slow if you apply only the changes for the Tilemap Component on it's own - if you go to the Overrides dropdown and use "Apply All", it is a lot faster.

1. Install Unity
2. Open the attached Project
3. Open TilemapScene.scene
4. Select the Grid Object in the Hierarchy
5. Go to Overrides in the Inspector and select the Tilemap object
6. Select apply > apply to prefab

Result:
It takes quite a while to apply the changes to the prefab.

Expected Result:
Changes should be applied as fast as when Apply All is used in the Overrides dropdown.