A T1 belt throughput is 15 items/s. Player inventory is 80 slots (2 cargo wagons, incidentally, though players load/unload instantly).

Stack size varies, but let's take 75 as an average, which means a player can carry 5600 items.

But carrying isn't throughput. If a round-trip takes 30s, player throughput will be 186 items/s. That's a measly 12 T1 belts.