Perhaps part of the success shooting black powder today is the Big Lube and similar bullets available, reloaders are using much more lube and even grease cookies to keep the fouling softer. These heavily lubed cartridges were not available or issued back in the day. I don't know what lube the early cartridges used, but the lube grooves of the early bullets don't look much bigger than modern bullets made foe smokeless. Going back even further, I don't think the paper cartridges for cap & ball contained any lube at all, or not much, if any, so they must have fouled quickly as well.
For the ordinary, non-military gun owner a century and more ago, they didn't fire enough rounds in an incident to foul a gun to a stop, normally. If just target shooting, they were probably content to clean the revolver often enough, and if confronted by a snake or other beast, or hunting, probably only fired a few shots at most.
Other than the lack of similar buildup of fouling, the subs don't seem to be any slipperier in their fouling than real black powder, I found 777 or Ascorbic Acid powders to be pretty sticky too, not like smokeless.