For my friends uni project I have to run at different speeds on a treadmill (treadmills are crap!), then I have to do uphill as well which is also fast vs slow. As far as I am concerned running slowly is a lot harder, then I run fast my muscles contract very quickly and then are able to relax very quickly, where as when I run slow, I have to slow contract my muscles and maintain the contraction for quite a long time before I can start to relax.
I have to run in 30 minute intervals, I have done the slow three times, fast three times and the uphill twice, on the fast run, I can go running a lot longer than 30 minutes no problem, on the uphill, the fast uphill is pretty easy as well, for me fast uphill is easier than slow on the flat. My heart rate is highest on the slow runs as well, which means my body is working harder, so I will be burning more calories and gaining more muscle tone on my slow runs, compared to my fast runs.
If anyone ever asks you to help them do research for their dissertation, say no!