I, for one, don't think Ultimecia was going to compress all of time, only a sizeable chunk in the area surrounding her world. Enough to give her basically absolute power, anyway, especially in the eyes of the world's inhabitants. [q=Viator]Time has many infinite time dimensions
The number of time dimensions is infinite. This means that you cannot count how many time dimensions as it goes on forever.[/q]The number of dimensions has no relevance to the size of something. A one-dimensional form - a line - could, in theory, be infinitely long. There's some speculation that time itself IS just a single dimension, much like height and the other physical dimensions.

Ultimecia's plan, it seems, was to compress the matter and energy from across the ages into a single focal point - herself. The first stages of that process enabled Squall and others to cross into her time, as the various eras became intermingled and less separated from one another. [q=Viator]Time slows down the faster you move.
I wonder if this just a theory or for real.[/q]Since this actually happens in the real world, and can be observed through existing technolohy, then yes - it's real. It's a fundamental factor in understanding the malleability of space and time.