Changing the speed of the TTA from how it operates now would be a fairly involved undertaking. It's not just a simple matter of flipping a few switches or re-programming a ride control computer.
Every time the trains speed up or slow down, there is a section of very-closely-spaced LIMs on the track. (This is most easily seen in the section of track at the start of the ride just after the station. When a train exiting the station hits that tight group of LIMs, it speeds up and pulls away from the pack in the station)
These close-together LIMs are needed to "force" the train from one speed to another (slow-to-fast or fast-to-slow). Outside of these speed-change areas are evenly-spaced LIMs that simply work to keep the trains moving at whatever constant speed they're supposed to. If they didn't have these speed-change groups of LIMs and just went from stretches of "slow" LIMs to stretches of "fast" LIMs, the trains would eventually speed up and slow down, but it would take a much longer distance for them to do it.
So if you wanted to change the location where a train sped up, it would require a major change to the already-installed LIMs. Say, for example, you wanted to keep the trains moving at the slow speed all through the Mountain until the trains went outside. They would have to remove the line of "speed up" LIMs from above the queue area, re-install them farther down the track, and then change the "constant speed" LIMs along the way to operate at a slow speed. And at slow speeds the LIMs probably need to be closer together, so they'd have to install even more LIMs and at shorter intervals. Throw in all the electrical work for such an undertaking, and you're talking some decent money.
Though one of the pluses of doing this, it would allow them to increase capacity by a couple trains, as the total ride-time would increase.
-Rob