Changing Speed of a Time Lapse Sequence

At age 66, I have become like a "kid in a candy store" after discovering that my Nikon D5300 makes taking time lapse sequences a piece of cake.

I import the pictures into iPhoto, create the event, and then drag them into a project.

I have found where I can adjust the time between each "frame" to no less than 0.1 of a second, but I want the movie frame rate to be faster.

Has anyone found out how to do this?

I know I can take the original picture with a larger gap between shots, but that way you lose time detail in the subject being shot. I would actually like to take more detail (for example, only a 3 second gap between shots, rather than 7 or 10 seconds) but need to find a way to speed up the movie.

Very grateful for any input on my NEW experiment in photography/video.