I remember 3 years ago I moved houses and had 1 jack with really bad performance. Looked in the attic and found that somebody had (for no reason) spliced the cable. With wire nuts. 1 per wire.
I did an Y-splice for three computers, without switch/hub, it worked.
I saw a damaged link with 50% loss on 3com 905 on the server end, RDP was unusable. Replaced with RTL8139 (other side was the same lost to time noname unmanaged switch) - less than 5% loss with ICMP ping. RDP worked fine.