It's not that slow--even the bigger bullets are going around 2700 fps. The problem with "let it foul" is allowing a build-up to trap potential rust /corrosion action by attracting moisture over time--which is exactly what I think happened to this rifle. It can also be hard to tell just how much build-up there is; I've cleaned barrels many times and was fooled by the color of the bore when I thought the job was done, only to break through to a new layer of shiny copper.
Holding tight and pulling in fairly hard is a matter of self-preservation with the bigger recoiling cartridges in my experience, you're probably going to get hurt if you don't. This Weatherby Mark 5 is very well made and balanced IMO, around 9 pounds weight with meat in the right places to get a really good hold and shoot steady. Keeping the recoil impact from getting a "running start" is key to making the felt recoil very tolerable IMO--along with the Accubrake which I'm sure helps helps reduce muzzle jump as well. I genuinely enjoy shooting this rifle, making big holes with big bullets going fast is kinda fun (though expensive).