Ultimecia CANNOT travel through time directly. That was kind of the reason she didn't just transfer herself back in time, right to where Squall is sitting on the can defenseless, and smite him there and then (I note Squall specifically cause apparently, he's the one she wants dead specifically, being the "legendary SeeD" who is to face her in an indefinite Time Loop and always kick her ass). Her powers can transcend time, as evident in how she can control Edea and Rinoa, but she herself cannot traverse time. The Time Compression was basically a way to cheat time and her mortal life span (supposedly, Ultimania confirms that all Sorceresses, Ultimecia included, have normal human life spans, regardless of how powerful they are, so she's by no means immortal either) and what not. She herself has limited control over time.
Its also worth noting that Ultimecia only managed to compress time cause the HEROES LET HER. That was kind of a gambit on their end; let Ultimecia take control of Rinoa again, and make use of Ellone's powers, to somehow speed up Ultimecia's methods, which would allow them and Ultimecia to break the boundaries of time and finally face off. Its clear Time Compression is not a simple case of "Snaps fingers, TIME IS COMPRESSED!" Ultimecia was clearly trying to pull this off at every moment, and it obviously required a lot of work.
FURTHERMORE, I seem to recall that for Ultimecia to successfully compress time, she NEEDS Ellone. That's why she was taking control of Sorcerress' in that time specifically. Without Ellone factor in, Ultimecia's just another Sorceress, if an exceptional one (she did, after all, create a GF just by reading Squall's mind, so she's clearly talented to say the least.)
So yeah, Ultimecia's Control Over Time is a bit overrated. Though, if Dissidia is any indication, she can Freeze Time for limited spans, evident in her fight with Squall, where he slashes her, and she sort of just fades out of the way to another spot instantly, and we even see her freeze time on the spot. Though, at the same time, Dissidia seems to imply she cannot actually attack until she starts time up again (she can prepare her moves it seems, but they won't do anything until she starts up time again. She creates a crap load of spears around Squall, but none of them fire off until she has time act up again.)
I know, Dissidia is not the best of things to bank on, but when you have plot powers as vague as Ultimecia's, it may very well be the best source we have :\