Mano-Wii committed what looks like a major speedup for vertex drawing in edit mode.
https://developer.blender.org/rB1f1da268402f241d83417ea0cc04ef74d0f9097f
To sum up, drawing time in some cases related to overlays was cut by more than half (in this case, it was in the situation where you have a subsurf modifier and an edit cage).
EDIT: Another one (though with a smaller time reduction). Benched with editmode cages on meshes with modifiers.
https://developer.blender.org/rB41ad845531b77f14c7836f5d09be50ea8b51c088