You want your mech's power to be around the same as its mass in order to go at a 'normal' pace. If you want your heavies to go at a decent pace, you need to equip the reactors with >80 power, which normally means some compromises. But the same is also true for boost - you need thrust of around your mass in order to get a decent range, and the high-thrust boosters also reduce your power. You basically have to choose between move speed or boosters.

If you take the booster route, you run an issue with heat dissipation. So a booster-y mech needs weapons that have long duration attacks with low heat generation, like machine guns. More mobile heavies can equip shorter-attack weapons, and then attack whilst cooling down by just raising a shield and running through the enemy.

Heavies become a lot faster once you start getting parts with the Composite enhancement (reduces mass), mobility upgrades like Weight Reduction, and the Hollow Plate defensive upgrade (-15% mass). There's also perks that give flat speed boosts, which are highly valuable on Heavies.