This completely screw up my theory. The adapter has Realtek chip onboard.
Therefore, question become:
Why this kind of card has crappy performance?
Indeed, that's not a news. Searching for "82541PI slow network" plenty of linux installations/distros seems to not manage quite well this kind of chipset.
Some posts reported 10mbit negotioation with the switch, others a sort of unreliability of the connection (falls, come back online, stays online 3-4 secs, then falls again).