If it only affects 10Gbe NICs it sounds like something can't handle receiving at that data rate and drops packets. This would trigger a backoff in transmission rates. This should converge at something near the available bandwidth, but it sounds like your congestion control is screwed up. You might also check your ethernet flow control settings and make sure they're consistent. If they're all currently on turn them off, and vice versa. (Flow control is often a bad idea, but sometimes helps.)