It's my opinion that in nature, one doesn't have to be fast. One just has to be fast
enough.
BTW did that make them any less agile?
Anyway, interestingly enough
http://theropods.blogspot.com/2011/04/theropod-speed-and-locomotion-part-ii.html
Put Deinonychus at 30-40Mph. That is four time plus faster than most people.
Not slow.
Now the bigger you get the slower you go, it puts Giga- at 10ish Mph. However one wonders how fast Argentinasaurus moved. My bet is that it wasn't all that zippy.