Further study shows there is a progressive linear degradation in exported frames per second proportional to clip length. This happens on rendered or non-rendered timelines. It may be related to a mask validation system by which FCP examines the entire clip's worth of analysis records to make decisions about the current frame.
This is normally not a problem since most timelines don't have 10+ minute unedited clips. However some instructional or event videos may have long clips. If magnetic mask is applied to a clip over 5-to-10 minutes long, render performance, export performance and to some degree editing performance will degrade.
Magnetic Mask relative render/export performance cases on a 30-minute timeline composed of varying clip lengths:
For a timeline composed of one 30-minute clip, vs a timeline composed of six 5-minute clips, the 30-minute timeline will render or export 5.4 times slower.
For a 30-minute timeline composed of three 10-minute clips, the timeline will render or export 2.7 times slower than a 30-minute timeline composed of six 5-minute clips.
If the timeline is fully rendered to cache, the magnetic mask will nonetheless be recomputed for export. That might be a separate issue involving cache management.
For clips 5 min or shorter, if magnetic mask is applied, the effect is very fast, and may not require rendering for good editing performance. The full timeline export performance will be much faster if magnetic mask is used on 5-minute or shorter clips -- even if the total timeline length subjected to magnetic mask is the same. Editing performance will also be faster.
For timelines consisting of long instructional or event material, you can simply blade the clip every 5 min, apply the mask and analyze it. I've seen no mask-tracking discrepancies at the boundary of a "through edit."
