Peter,
This has been a known issue that many have experienced on this forum and have suggested improvements. The best work around to slow it down is to produce to a file and then bring the produced video into a new project and then slow it down.
1) create magic movie format as you wish, usually without music
2) produce to a file
3) load produced file back into PD7
4) adjust speed of your imported produced video file and add music if you wish
Jeff