Here is why it is slow. When you output to compressor from FCP, compressor ignores all render files and creates new ones that are more compatible with the output format. So, for instance, if you do a project and hit render, and it takes an hour to render, then compressor will take an hour to re-render, plus whatever time it taks to actually compress the video. If you export your movie as a self contained file, and bring that into compressor, the time will be much less.
However, I still agree using QT in FCP is much quicker.