If you want to do slow motion, shoot interlaced, even if your final project is going to be progressive. It is more important to have twice the samples per second than full vertical resolution.
Compressor is better than using the Clip Speed feature of Final Cut – if the footage is interlaced, FCP throws a field away first before doing any frame blending.
Export your clip as a QuickTime movie.
If you want half-speed, in the Frame Controls tab, you only need use frame blending because Compressor turns each field into a frame:
If you want to slow down further, set the duration to be longer, you’ll need to use Motion Compensated retiming:
This will take a long time to compress, but should produce good results.