game_text Scan Out
After reading this I fiddled about with it in Hammer.
Set your Fade in Time (or character scan time) to something like... 0.2?
That's apparently the property that affects how long the individual letters will take to "scan".
Fade Out Time is pretty self-explanatory, but to be clear, how long it takes to fade out. 
Hold Time is how long the text should just "sit" there before starting the fade out process, after the fade in/scan in is complete.
Might be some more 'pro' tips, but that's what I could find out after 5 minutes and 2 map compiles.
*edit
After some more fiddling, I found that the following settings worked best (for my taste)
Fade in Time: 0.05
Fade out time: 0.5
Holdt time: 2
Scan time: 0.5
[Scan time is affects how long the scan effect displays.]