I agree that 30 seconds each is pretty reasonable. I mounted a ton of 'em back in the 80s and early 90s when I shot a lot of E6. I too prefer the plastic slide in mounts, which make them easy to remove. That was important to me then because I was printing type R (and later, when I could afford it, Cibachrome) prints from them and didn't have a mounted slide holder. Other types may hold them flatter, however.

My experience is limited to 35mm slides. I would like to get a MF projector and shoot E6 in my 645 and 6x6 cameras but so far I haven't (well I've shot some E6, don't have the MF projector so they are still unmounted.)