I will second bill on the cable.
I also had done some continuity testing on cat5(e?) cables that looked ok and passed. When trying to move a file from one system to another on the lan which would usually take 5min on other cables, the same transfer was taking over 3x as long.
Proof to me was starting the transfer with the tested cable - eta to complete of ~15min, unplug the cable, which caused the transfer to fail with option to retry, swap to known good cable, hit retry, and transfer would complete in under 5min.