Just an idea. Have you already patched and removed the limit on TCP connection attempts in the tcpip.sys file in your Windows XP installation?
And don't forget to adjust your MTU and RWIN values as Windows XP does not do it automatically! This can be easily done, for example, with the tool SpeedGuide.net TCP Optimizer, currently in the version 4.1.1. The correct MTU value mainly depends on your internet provider and can even be determined with the SG TCP Optimizer tool in the corresponding tab. A suitable RWIN value can also be manually calculated and set in the registry, of course. All values should be then tested to find the optimal ones. In any case, be aware that such fast connection speeds as you are using today didn't exist when Windows XP was developed and still maintained. TBH, I don't know whether Windows XP with its very old network drivers is at all able to handle such high speed you mentioned above. The old Internet tariff I booked years ago has a speed of 25 Mbit/s only, which I can achieve in full on my Windows XP computer, but only with optimal settings mentioned above. However, my old processor wouldn't be able to cope with any more, either. And regarding the QoS Packet Scheduler, I deactivated it from the very first on my standalone Windows XP desktop computer long, long time ago.