Unusual problem on my LAN that affects my ADSL Internet performance.

When I run a speedtest on my Windows 10 PC, it affects my ADSL2 router WAN performance. The other non-Win10 machines on the network show slow Speedtests until the Win10 machine is rebooted.

I have the following config

Router: TP-Link TL-8960n (ADSL2) - Also tried on a TP-Link 8840 (ADSL2)

TP-Link Gigabit switches and Belkin 10/1000 switches

Static IP: DHCP disabled except for a very small range with MAC addresses assigned to that range.

Wireless access points: a range of TP-Link 802.11n wireless access points

Mixture of Windows 10, Vista, FreeBSD, Raspbian, iOS on the network

Up until recently this was all working fine, then I rebooted my Raspberry Pi (which has always had difficulty connecting via wifi, but did have an uptime of 74 days). Since then the network has been unstable with ADSL connectivity dropping. However, even with the Raspberry Pi off I can re-create an issue where running a speedtest from the windows 10 machine somehow seems to "contaminate" the ADSL performance until that win10 machine is rebooted.

Weird.