Going through Austin can add another hour to the trip at some times of the day. I suppose one could bypass Austin and go 85 mph on the brand new TX-130 toll road.
Or 220 mph: Hennessey VR1200 Runs 220.5 MPH on Texas Toll Road
Seriously, I've taken that bypass. It's 11.5 miles farther than straight through on I-35. At posted speeds, I-35 is 4 minutes shorter. But, you can rarely drive the posted speeds through I-35.