wired Internet OK, WIFI signal STRONG, but WIFI internet DEAD SLOW - any ideas?

[#1]

Hello all,
I'm having a problem I don't understand, please can you help?
My WIFI internet is so slow it's unusable for simple web-browsing and filling in forms.
This applies both on a laptop, and on a tablet, so don't think the problem is with the device.
1. My internet connection is ok.  I've speed tested it from my laptop connected direct by cable to the router.  (ping  20ms, DL = 7Mbps)
2. My WIFI connection is showing a full strength signal
BUT  my WIFI internet is still useless....  using a speed test gives ping at over 1000ms,  DL at a few kbps
Please can anyone tell me what the problem is?
(and how to fix it!)
Thanks
Simon

[#2]

Have you tried changing the WiFi channel?

[#3]

Hi there, if you go and download "acrylic wifi home" its freeware and install then open you will see all wifi signals in range and what channel they are on change yours to either 1, 6 or 11 (least interference with other channels) use the furthest away channel from any other signal you see and see if that improves throughput