If the gunk on the shutter blades is any sort of oily residue it must be removed.
In practice flush cleaning WILL solve a multitude of issues. You're going to find some recent threads poo pooing this method, and it's not as good as disassembling and proper cleaning.
FWIW typical tolerances on these critters are +/- 20% at slower speeds and 25% or more at faster ones.