Or to 10mbps, half duplex. I’ve witnessed this. My former company was trying to sell a client a new server because it was too slow when I noticed it was only operating at 10/half, instead of the 1000/full that both it and the switch was capable of. Some testing later, and the problem was at the server side cable termination, a quick re-termination and they were up to gigabit. Grabbed a spare run to the switch and connected another cable after verifying it was good and the company went from 10M/half to a LAG of 2000/full in the matter of about an hour.

The speed complaints stopped.