@Geto 

Swap the cable you are using from your computer to the S4.

Go into the QoS settings and make sure the bandwidth is set to something like 1000 download and 1000 upload (manually set it).

Do the speed test on your computer in safe mode. It seems odd, but sometimes the computer can have software affecting speed in connecting to the modem or to the router. I think the previous case involved a Dell computer and some software it came with.

@Ezuzu 

No, if those basic steps do not work I would then reach out to support to see what the cause may be.

@Tony I've got in touch with them lets see what they say. As this is quite annoying the fact I'm only getting barly a 3rd of the speed I'm paying for. 

@Geto Experiencing the same problem. I get 350mbps down at 12 up (consistently) when connected to the Comcast Modem via cable OR Wi-Fi, BUT when I'm on the Mesh WiFi network, I get at most 160mbps down and 12 up. I've tried moving them to different places around the house and no changes, I'm about to contact support, and if they don't have a solution... I WILL be returning this for a refund, for a better quality Mesh WiFi Network.