I've come across and issue with the Firestick's (Gen 2) interaction with VPN's. When a vpn is turned on, the wifi speeds of the firstick get reduced to between 13-18Mbs. It doesn't matter what the wifi speeds are prior to the vpn being turned on (the same result was seen from Wifi speeds of 50Mbs to 80Mbs). This is beyond normal speed loss for VPN's. To test, I tried the same network with a Fire TV Box (Gen 2). There was virtually no speed loss at all over Wifi when the VPN was turned on.
Variables I tried: 3 different paid VPN's, 3 different ISP's, Owned vs Rented Routers, I restarted the routers and even tried port forwarding to see if it was an ISP issue, but it made no difference.
Something (hardware or software related) on the firestick is triggering a slow down when a vpn is turned on. It could be the processor, memory or wifi chip set up as those are the two major differences between the fire tv box and stick.
Is this a known issue? Before asking if I tried to do x or y... I did. I've tried everything from router settings to different routers to different ISP's to different VPN's and settings. Nothing worked. I don't believe it's in the FireOS because the FireTV box doesn't have this issue.