SHared IRQ for network card
Yes, it is possible for a shared IRQ to slowdown a device. Really, you have to experiment. Change the IRQ setting by moving the card to a different PCI slot. Test the card on each slot to see which one will give you the best performane (If it's shared with another IRQ). Of course, if you find a slot where there is nothing shared, you wont have to go through all of that. Just a suggestion.