But I don't the GP comment was talking about that; I have also used WiFi without an antenna (PCB or otherwise), and it was also slow for me.
A dummy load is probably the best idea: an impedance mismatch would obliterate any chance of anything working (including Bluetooth fwiw).
If you know what you are doing and have explicitly disconnected/cut all antennas including entire MIMO arrays (many people miss one or more antennas) in the motherboard then the only remaining explanation is "accidental antennas".
This is only possible if the WiFi-chip has dangling metal pins/traces/joints whose length is comparable to that of its wavelength, in which case these will act as makeshift antennas. These will pickup ambient WiFi RF, but for this to work at all you need lots of RF power i.e. a very strong signal strength which means you have to be very close to the WiFi router.
The whole thing works because of Physics and not Engineering (the "accidental" part) and hence very iffy (may not work at all) and should not be relied upon.