Were you doing anything else on the computer or internet connection at the time ?
I get a similar thing if I try to run 3 clients simultaneously - 2 works OK, but the 3rd pushes my graphics card over the ‘knee of the curve’ and drops my FPS from 50 to 3. The slowdown doesn’t resolve if I just stop 1 or 2 clients, I have to stop all 3 and then restart (2 of) them.
Silly as that sounds, it does happen. I used to look after my employers mini-computers (HP3000). After one or two ‘incidents’ local staff were told to leave them strictly alone and call for support (me) if they went wrong. So one day I get a call that the machine in Dublin keeps crashing. Flew over (from the UK), went to the site, entered the computer room, opened the cabinet - eh voila, a large mice nest ! They’d eaten the insulation off some of the mains wiring… I arranged for pest-control and a hardware engineer to visit, then flew back the same day. I dread to think what that little jaunt cost the company, but I did enjoy the day out.