Hey Barb. tjkraz is right about the parade drawing folks away from the attractions. This is true in every park, not just the Studios. 3 pm is a decent time to ride the non-FASTPASS attractions in Tomorrowland, for example. And I think tjkraz meant to say 'AK' not 'MK' - the AK exhibits a steep drop-off in crowds after 3:30 pm. At least until Everest opens.
During summer, I think that thinning crowds will be, relatively speaking, still big at the Studios. And as AuroraluvsPhillip notes, the FASTPASSes for the most popular attractions will probably be gone, or really far into the night.
If you got to the Studios by 1 or 1:30 pm, you'll be okay. Much later than that and I'd say it's probably not worth the effort.
Dansyr, I'd consider those dates 'fall' - they're less crowded than summer.
Hope this helps,
Len