While the Covenant may manuever faster due to the Slipspace-short-jump capabilities, with it's obvious drawbacks ignored for a moment, it doesn't help them too much, as the Imperium outranges them by... Well, Imperial Standard Armament (Macrocannons, Laser Batteries, etc) Extreme Range is in the 70,000 - 100,000 km, with 30-60,000 km being optimal range. ...The Covenant engage, at most, around 10,000 km, with a few rare exceptions like the Sniper Flagship/Battlecruiser that shows up in The Fall of Reach that has a range of (approximated) 100,000 km. (1 in the 314 ship strong invasion force that they pointed at Reach, the assumed Human Homeworld. They have another 2000-3000 ships with High Charity, so they could have (going by that ratio) a whole... 10? More, if you assume they keep the better ships at home.)
Covenant Fighters might be superior to Imperial, but it's difficult to tell, as all they ever fight is Longswords, and go about even against them.
And all the Reclaimers help them if there's an "outdated*" Nova Cannon on a cruiser. Fires rounds at >.75C, that implode hard enough to hurt Imperium-sized/armoured ships when they miss by a few hundred kilometers. There are a variety of Nova Cannons in fluff, as usual for 40K, some hilariously weaker than this, some hilariously stronger.
*Technically, it's still a marvel of engineering for the Imperium, they just consider it outdated in the Military Tactic sense, having moved Prow-Shooting > Horde of Fighters > Broadsides > ???, depending on Sector.
** Lances, while blocked by Imperium Void Shields, have the descriptor Ignores Armour. These buggers can lance straight through ships if they don't have Void Shields up.