Having driven both versions of the road, my gut feeling is that the old Katy wasn’t always jammed, but when it was it was bad. Whereas the new Katy is just sort of uniformly slow during peak hours.
Rail transit journeys are often somewhat longer than the equivalent auto-trip at free flow. Rail proponents tend to argue that “you don’t have to worry about traffic jams” – that the predictability offsets the increased time – and I suspect something similar may be happening along IH-10.
From a highway engineer’s perspective, a road that runs at 70mph at night and crawls along at a consistent 30-40mph during the rush is absolutely ideal. It means the rush-hour folks are getting maximum usage out of the capacity. What we don’t want to see is random breakdowns that cascade into start-stop jams that take hours to clear. We also don’t really want to see roads running at 75mph during the rush, because that means we probably should’ve used that concrete somewhere else.
I also get the impression that the Katy traffic is actually worse west of SH 6 (and approaching SH 6 from the east in the PM), which suggests that the managed lanes should have been continued further west.