Alright I think I've found the solution to this one.
WAN > Edit PVC > Enable 802.1Q.
Why the hell does VLAN packet info affect wifi performance for only a windows laptop? At least it works..
I have a brand new DSL-AC86U which appears to be throttling or otherwise slowing the wifi connection only to a windows laptop. (I have no other PCs connected or at hand, just android phones)
Firmware version: ASUS 3.0.0.4.386_39648
The VDSL connection is 25mbit/5.
Multiple android phones have...