My cataphract does 80kph, only marginally faster than the 4 x.
Part of the reason, I believe, has to do with the metagame. The 4x has a great layout, hardpoint-wise, and if you spead it up, it would compete with the Ilya, and they wouldn't make any money from selling it. What I mean, by the metagame, is that they don't want any mech to be the best of all things or people would only play certain mechs. (They are failing at this with ECM equipped mechs, but are generally successful with the rest of them.)
HOWEVER
There is a great deal of ill-logic going on here. I think the Awesome is the best example. The 9M, which looks structurally identical to the 8Q, can fit a much larger engine inside and this makes no sense at all. The ONLY explanation is the metagame - game balance. I really, really miss my fast 8Q laserboat and I'm sure those who used to use fast laserboat hunchbacks really miss them too. The devs felt these mechs were overpowered with such speed possible. For the same reason, the 4X is as slow as an Atlas. I guess one thing that really bugs me about the Awesome (most of them), especially now with the Stalkers out, is that an 80 ton mech cannot go any faster than an Atlas. This was OK in tabletop, because it was cheaper to field an awesome than an atlas, and you had dropweight restrictions/scenario restraints that explained it. Here, the Atlas's have just as good, or better, hardpoint layouts than the Awesomes, similar performance characteristics, be can hold 20 more tons of armor, weapons, and heatsinks. So WHY would we take an Awesome when we get so much more in an Atlas or a Stalker?
But maybe they'll make a hero version of the Awesome. I vote for the Awesome 9Q. It's a little early in the timeline for it, but all the technology is already exists now. 4 ppc's, big engine and an ECM. Now, THAT would be a fun mech!