DVDLab takes a text based sub file and then generates a precompiled graphic subpicture type file thingey (sp1, sp2, sp3, etc.)
I don't know of anything that works with sp# files, you'd need the original subrip/text based subs to retime.
I'm sure opensubtitles.org or subscene should have something you can use if you don't have the raw file.