I did notice slowdowns when IR is running in Corona 13. Slowdowns are not in the viewport, but rather in slate material editor. When IR is running everything in SME is more sluggish - dragging nodes around, connecting node handles, almost every operation needs to be tried 2-3 times before succeeding. Not a deal breaker, but certainly adds up to quite frustrating experience. SME was never smooth in Corona, but in V13 it's even worse than usual.