Par4QC High scores do not equate to slow play.
Not always and there are no absolutes......however, I can say with conviction that if I have a slow group, it is bad golfers and/or over 75 year olds who walk 7-8/10 times. These groups could be searching for multiple balls on every hole and taking an average of 7-8 shots a hole.
The other 2-3/10 its gamblers, on Ladies day a group of middle aged women who are out for a stroll , a group of juniors or the millennials mentioned earlier. Most days are good and as said, most people are good about it. Not uncommon to get, I paid my money can take my time, I usually say, so did the 200 other folks and we have to keep things moving reasonably for all.
At our course marshals get let go for shagging balls but we have a ball bin at barn so I grab a couple dozen before every shift and hand them out on the course for those groups who are persistently searching. We have a fair bit of fescue and a creek that winds through all 18 holes so it helps.
that said, I play with the other Marshals who are mostly in their late 60's and 70's , we ride and can get around in 3 or less hours ( generally in the first couple groups out) I play most of my golf after dinner and can often get around all 18 in a cart in 2 hours