In UE 5.4, the performance of the editor UI has regressed severely. Anytime a mesh in the level is selected, for which the thumbnails were not generated yet, the material section in the properties panel will flicker once for each material slot, freezing the UI in the meanwhile. It looks ugly and makes the editor laggy. It was not the case in UE 5.3 and prior, so it’s a regression.
It’d be a shame if Unreal Editor became yet another slow, laggy application like 3ds Max 