I find that the more lead fouling that accumulates the worse off my accuracy becomes.
At 50 yards it's not too noticeable but at 100 yards, the fliers in my groups become more evident.
Starting with clean barrel, the first 10 slugs all group pretty consistantly but once I get to my 20th or so slug shot, the lead fouling is apparent and accuracy drops off some.
I just posted an example video of this on Youtube if you care to see what I'm finding. The first shot is a bad hold on my part and shoots high left. Shots 2, 3 and 4 are all good holds but the fourth shot falls short and left. I believe this to be due to lead fouling in the barrel.
https://youtu.be/7aCbU8Lxbb8