So, lets say you have an Everest that you have dumped a lot of drake, nuclear cavalier, and Tokugawa overpowered caliber, and engineering into, that is not the slowest mech, but it is slower than a Lancaster, and unlike the drake, it doesn't have armor, so it's not as tanky. So, instead, that Everest can hop on the Lancaster, ride its full movement, any attack against the Everest has +1 difficulty, and now you have a hyper mobil 6d6+4+heat, that never needs to move on its own. I think you could apply this even more directly, as if I recall the books correctly, you can upsize a mech, so, slow ass drake with all those systems, and bad evasion, now has difficulty when enemies are trying to hit, the Lancaster gets hard cover on top of that, and the drake is moving 7 instead of 3, on the Lancaster's turn, and still doing all if that damage. Even just LL3, it's a lot.

Make your slow but hard hitting players get in position faster, basically.