How do I compose something fast with this?

First things first, the time signature only measures how many beats are in a bar, not the actual speed of the song. You could set the time signature from 4/4 to something as obscene as 43/128 time and it'll still just be that same moderate tempo. Here's how you can make your song fast:

Well, to increase (or decrease) the tempo simply select the first note, rest, whatever on the file and then click on the "create" bar at the top of the screen. Once you are at it, scroll down to "Text". At "text", more options should pop up. Scroll down to "tempo" and it opens a menu showing all sorts of sample tempos you could use, the slowest being 48BPM and the fastest being 208BPM. Simply click on any pre-set tempos or you can type in your own text/ BPM to make a custom tempo. You can also access this menu by hitting ctrl+alt+t (Not sure what it is on Mac, though. That's just for PC.)

If you want to change the tempo in the middle of the song, simply select the rest/ note where you want the tempo to increase and then follow the same procedure.

Hope that helps!

Also be sure to add notations to the score at the appropriate points, to tell "the humans" when to speed up their playing, and to do so exactly where you have just told "the computer" to speed up. Because the two sets of directions are not the same.