With my rifles it has varied from barrel to barrel, but eventually at some point with every barrel I will need to remove at least some of the copper fouling to bring back the performance. I don't worry anymore about removing all the copper. I simply spend some time with a dedicated copper solvent and remove most of the copper.

FYI, if it is the copper causing problems you will need to let the solvent do most of the work. The patch is just the delivery system. Bore snakes don't really do anything at all for copper removal.