The method Tim used is working fine here in R15 but it does seem that you can't animate the cache scale parameter. As Tim suggests, you can use a time track on the backed animation if you want to vary the speed.
In the file I created, a quite complex cloth explosion, for some reason the playback in the editor of the time track gives some errors. The animation feedback is really choppy and not doing what it is supposed to do. Is this a normal thing?
Okay, a couple issues. First, it looks like you're applying the time track to the dynamic tag's pressure animation track rather than applying it to baked PLA data. That's certainly something you can do, but I think it's a lot harder to predict the results that way than it is if you bake the dynamics to PLA keyframes first and then adjust the PLA timing.
Second, your time track slants up and then slopes down for a bit before sloping back up. Not sure if you realize it, but when a time track slopes down it will actually cause the animation to run in reverse.
Your file is a little hard to parse because of all the display tags, but if I was you I'd first bake the dynamics tags (use bake all) in the tag's cache tab, then the bake function in the timeline. Then I'd bake the cloth tags to PLA (don't have to cache first). Then I would delete all the cloth and dynamics tags and apply a single time track to all the baked geometry.
I have been playing around trying all possibilities, and everytime I bake the objects and remove the tags, the objects behave really badly.
In the first place, I tried to follow your steps. In the first place if I press Bake All in the dynamics tag, the square bounce really heavy. Later applying the bake function, kind of works, but not doing exactly the same ( do you bake the soft body dynamics to PLA too?) . I baked the cloth tags to PLA and after I delete them, they explosion goes crazy splashing polygons all over the place.