On the contrary, when I worked at reddit, we found that people allocate a certain amount of time to using the site (consciously or subconsciously) and the speed of the site doesn't seem to have much effect on that (unless it's completely unresponsive).

In other words, if they can read 100 threads in 15 minutes because things are running really smoothly, they will. If they can only read 50 threads in 15 minutes because things are under heavy load, then we would just get 50 pageviews from them.

TLDR: Slow reddit -> fewer pageviews, same time on site