I'm shooting hornady 240gr swaged bullets in .44 sp. at less than 800fps (650-700fps range) with no leading problems. Also some 200gr 20/1 at less than 800fps. These both shoot better than hard cast. I believe I was having trouble with hard cast because of the beveled edge....lots of leading.
I'm shooting .454's in a Colt New service in 20/1 as well. Leading problems with it are over as well, Started out with .452's and leading was horrific. In this case I found the older Colts were using a larger bore back before WWII.
I general, I find any lead bullet, either hard cast or soft with a beveled base to be an invitation to leading no matter how hard or slow I push it, probably just me.... As others have stated, proper bullet to bore size is very important. I slug everything these days before I start loading for it.