Just created a test screen movie with QuickTime X 10.4. Unlike with movie recording, there do not appear to be any options for adjusting the quality of the movie in terms of resolution (unless you drag out a selection), frame rate, choice of codec, etc.
Screen recordings are recorded at 60 fps. When I look at the movie in MediaInfo, it shows frame rate 60 fps and original frame rate at 25 fps. Not exactly sure what this means but QuickTime X's Inspector and QuickTime 7 Pro's Inspector both report 60 fps.
One possible reason for your screen recording at 15 fps might be your hardware and memory were not up to the task. QuickTime X may have throttled down the frame rate based on available horsepower. Be sure to quit any applications you are not requiring be featured in the screen movie.
QuickTime 7 Pro does not have an editing function to change frame rate.