I'd just like to add that I have the same issue with my new Surface Pro 4. WiFi is terrible on 5 GHz unless I'm extremely close to the router. I have all of the latest updates installed.
Other devices on 5 GHz are fine in the same spot as the Surface. I'm showing 3/5 bars of signal, but getting 1-3 Mbps throughput.
My router at home is an ASUS RT-AC68P. I've tried at work and it's the same thing...if I'm right under an access point the WiFi is fast, otherwise it is awful. APs at work are Ruckus R710s.
It seems like the Surface's WiFi performance drops off significantly in medium to low signal situations. The problem is that I'm seeing the kind of performance I would expect at almost no signal when I still have a decent medium strength signal.
Hopefully this is just a driver issue. Although, I'm surprised since the chip is the same as the SP3.
Edited to add:
Looking at a few WiFi analyzers, I'm showing -58 to -60 dBm signal strength on the 5 GHz band when having issues with the Surface. This is within the "Good" range for WiFi signal, so definitely something going on with this chipset.