I have come across the same issue in a game made by my community.
Game: City of Kyiv - Roblox
If anyone knows anything that’d help a lot. We tried deleting all the meshparts, surfaceappearances, lighting objects, smoke objects, etc. Nothing helps. https://gyazo.com/c741d1699898423106234db3f7ecdd71
It is doing same thing as reported in this thread.
False alarm. We discovered a dev no longer on team must have placed a Humanoid object as child of Workspace which for some reason causes massive lag whenever Studio Select part tools are toggled on.