I've barely found it to be slow or sluggish. Heavy mechs may prolong the battle, but things can change quickly at any moment. I've played over hundreds of skirmishes and they have always been fast and exciting.
That doesn't change the fact that your medium of movement is slow as
forget from the perspective in the roosterpit. Those things are going
nowhere fast, and it's like playing in molasses. On the other hand, HAWKEN is like playing Counter-Strike in giant robots, which can be considered fast-paced, to an extent. The reasons behind this is because of the inclusion of dashes to maneuver in and out of cover rapidly, (if I recall correctly) whereas in Mechwarrior, once you're within the sights of somebody, you better be prepared to commit. Maneuvering is not an option in Mechwarrior.
In addition, level design in games is, surprise, important when it comes to determining whether a game is fast or slow-paced. There's a reason why HAWKEN can be considered fast-paced, and it's because there's actually cover to duck behind in that game. Mechwarrior generally doesn't have a whole lot of that, and why the forget would it. You'd poke your dumb head over the cover anyways. Armored Core's level design is also a mixture of both, having huge open spaces alongside areas of cover, and with the
maneuverability afforded to you in those games, you can take full advantage of ducking behind cover for brief moments to cause missiles to crash, or even just bounce back and forth using thrusters to avoid damage entirely. That is something that can be considered "fast-paced", because you are capable of rapid movement and retaliation through repositioning. Mechwarrior isn't fast-paced because it's literally
slow. Who the forget thinks it's fast.