The questioner wants plausible science fiction and there is plausible science to back it up. Let's not forget the time-bubble is constructed by an advanced alien race.
@celtschck suggested negative mass and negative energy, and quite correctly too. We may cast aside any objections about how to obtain negative matter and negative energy for the purposes of plausible science fiction and simply grant that our aliens are sufficiently advanced.
Robert L Forward described a negative matter time machine thus: "Suppose we had a negative matter which is very dense. Time would run faster near or in the negative mass and we could make a hollow sphere of dense negative mass to speed up time." (in Robert L Forward, "Far Out Physics", Analog, August 1975, pages 161 and 163).
This doesn't speed up time in an unlimited way,the speed-up factor is only square of two faster. This is roughly only forty percent faster. Forward proves this from basic gravitation equations which look like they're adapted from equations about the mass of black holes (my guess!). Forward's article isn't detailed like a scientific paper, alas.
Assuming special relativity is preserved inside a Forward-style dense negative-mass hollow sphere, then if time is faster lengths will be dilated so any measurements of lightspeed remain invariant as required by special relativity. Essentially as time is 'speeded-up' lengths inside the negative matter time machine will increase.
Like @Ruok2bu I had considered the possibility of a parallel universe where time is faster relative to that of our own universe. Yes, @JDLugosz, like Narnia.
Also various forms of time travel could be used to create an illusory version of speed-up time not just interesting arrangements of wormholes. A long space trip in a Godelian rotating universe would achieve in a similar result. The pity is rotating universes are always hard to find.
There is another way of creating speeded-up time. It involves special relativity and travelling at superluminal velocities. R T Jones published in the American Journal of Physics the possibility that travel faster than light results in time passing at the rate of the distance traversed. Basically for every light year a spacecraft travels one year passes shiptime.
So a FTL spacecraft only has to choose a suitable superluminal velocity to ensure enough speed-up time can pass. Since this is plausible science fiction, we can assume there is a chronology protection principle in this fictional universe to take care of any causality problems, Namely, there won't be any to worry about. This isn't exactly the time-bubble the OP wanted, but it is one way to speed up time.