[2023] Why the hell is M10 so sluggish?

The mobility gets better with the engine/transmission upgrades, yes. But, I would not use it to race towards the enemy. While it physically has a turret, it is among the slowest-turning in game. Trying to quickly turn and engage an enemy that has snuck up on you is usually a losing proposition from my experience.

Best played in the long range role and from the flanks/map borders.