The BaseXP payouts are much lower in Co-Op so FreeXP earnings in this game mode will come in at a trickle. The most glaring is FreeXPs, because it is based off of BaseXPs. But the payouts are significantly better for the time invested. Random Battles take a lot longer, 15 minute games has been the norm and it's not surprising to get the full 20 minutes. The games are over very quick so there's really no time to get an average with higher numbers for better rewards.

Typical Co-Op through no fault of your own, can bag you something like 200-400 BaseXPs. The reason is your rewards are often tied to how you perform, obviously. RIP if you're trying to grind CVs in Co-Op, you'll hate your life. When you get to High Tier, the grind via Co-Op gets much, much worse. Tier VII is where you start feeling it real good. Grinding in Co-Op is feasible until you get to Mid Tiers where the grind feels longer. The rewards in Co-Op are vastly lower compared to what you get in Random Battles. Is it faster to grind in coop? What are people doing?

I've historically grinded in randoms (with camo/flag exp boosts) since it gives more exp vs coop.

How to play ranked in world of warships