There are following bugs I am aware of:
1, One which drastically reduces Play in Editor performance. Introduced in 4.26, still present in 4.27 and is planned to be fixed in 5.0: Unreal Engine Issues and Bug Tracker (UE-108737) - You can tell if you are affected by this one by trying to play the game in standalone window. The performance degradation should not be there.
2, One which reduces Translucency performance. That one should not be present when you don’t have significant amount of translucent materials in your level. That one will affect standalone play mode as well as cooked builds.