yes, but I recommend using a good software for best result, like Final Cut Pro or Adobe After Effect (the one I use). You can try to do it in Quicktime Pro, but not recommanded.
If you just want to decrease frame rate by half or third, like (30->15) Quicktime is good. Just export your movie and select another frame rate.
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If you just want to decrease frame rate by half or third, like (30->15) Quicktime is good. Just export your movie and select another frame rate.