Full wifi signal but horrible download speeds
WiFi signal strength only tells just that, the strength of the signal.

It is a prerequisite to have a good connection.

How ever that does not tell you about the Quality.

If you have high interference (from devices or Neighboring networks) then your RSSI will be high. It should be &lt;70 or better (the lower the better).

Then there is the Transmit Rate value that will tell you the actual data transmit rate.

To look up all those values hold the option (alt) key while clicking on the WiFi Icon.