Sequence proxies are created using only 1 thread at the time.
Since this process is essential to effectively use video sequencer, performance can be deciding factor for users to use blender.
Progress bar will show progress for only one thread and since PC with 12 running threads is perfectly normal, my idea is to draw progress directly into sequence strips. Which may not be part of this diff, but I guess, we can discuss if this is good idea or not.
Updated diff to include changes to rendering progress in strip directly.
Job name is name of movie clip linked to sequence - personally I would create a function, that calculate path to proxy file, and use this path as a job name. But now all sequences with same movie clip are showing progress.
Some further modification of wm jobs manager.