Wait, don't other races use the same engines we do?
They do, but the consequences of reduced acceleration curves and heavily increased logistical restrictions also alters how combat plays out. Plasma carronades have rather low ranges for their tech level - for a 15cm beam at the fourth focusing/wavelength tech level, lasers can reach 240k km; a high-power microwave, rail gun or meson can hit something at only half that distance; and a 15cm plasma carronade, which has no range enhancements beyond its size, remains stuck at a paltry 60k. Basically, if all of the speeds are reduced proportionately, you'll still spend longer (and thus, spend longer taking fire) in any pursuit of a fleeing opponent that outranges you because the absolute speeds are still reduced, even if the relative proportions remain the same. That puts a stronger emphasis on gaining and keeping the range advantage, and further reduces the importance of low-range beams at tech levels before you hit the hard cap on beam weapons. For instance, this is pure napkin maths rather than in-game engine values, but if you're chasing an opponent traveling at half your speed with a 5000 km/s ship, you're gaining on them at a rate of 2500 km/s. If you're chasing an hostile ship with the same proportional speed to your own with a ship that can only manage 2500 km/s, you're only gaining at a rate of 1250 km/s, and will take hostile fire for twice as long to get into range of your own weapons if they use lasers and you're using HPMs since the weapons ranges did not change.
Still, that said, were carronades ever used much even in pre-6.2 games? Other than fixed jump-point defenses where the opponent is forced to jump in right on top of them, their extremely low ranges and extremely high damage reduction at range always made them a rather poor fit for mobile operations, I thought. HPMs were also always rather situational, like mesons, though mesons were more likely to end up in a useful situation (e.g., PD for dealing with armoured missiles). I still think effective early-game missile design is still possible in this as well, though point defense does increase in effectiveness in the early game compared to the older versions. It's a rather interesting strategic picture that's permitted by the new version, in my opinion.