There's nothing the XP computer can do to fix that, as it is the victim of the delays and needs the cooperation of other hosts that are to some degree involved

There's nothing the XP computer can do to fix that, as it is the victim of the delays and needs the cooperation of other hosts that are to some degree involved in causing the problem. As I understand it, ECN on the Mac enables the Mac to cooperate with queue management elsewhere (the Mac could be doing its own queue management without ECN, but probably doesn't). The Quake trouble indicates the bottleneck is an overlarge buffer on a router you both share. You need your shared router to do some queue management (CeroWrt is a good platform for this) so that large downloads don't increase delays for all connections. If your ISP modem has bloated buffers and you can't replace it, you may be entirely out of luck.