To me, it's 100% because of either their management at that particular stop or the workers they're attracting to that stop...I say that because the same Mcdonalds are almost always slow and the same mcdonalds are almost always fast, meaning if you find a fast mcdonalds it will likely always be fast in the future and if you find a slow one it's likely going to be slow forever.
I don't know why that is, but we used to make roadtrips along the interstate for 5 to 10 years, and without fail the fast ones stayed fast and the slow ones stayed slow.