I'm using a raspberry pi 2 model b running Ubuntu core, and a TP-Link TL-WN821Nv3 USB WiFi adapter, which has a maximum speed of 300mbps/37.5mBps, and a Samsung charger which supplies 2A (lower than the recommended 2.5A), Camera module is plugged too.
here are the results, ran from and with an open ssh connection:
user@localhost:~$ speed-test Ping 3600 ms Download 1.1 Mbps Upload 0.4 Mbps
I did a little test that didn't hands-down confirm this hypothesis: I plugged in mouse, when running the speed test this time, I kept moving the mouse held space key down periodically (so not to bottleneck with CPU/USB-controller), to ensure they're drawing current (100mA minimum each):
user@localhost:~$ speed-test Ping 3600 ms Download 1 Mbps Upload 0.3 Mbps
download and upload speed decreased by 0.1Mbps.