I often play CS:GO over WiFi, and while usually it's okay (I usually average about 60ms, which while isn't ideal is playable), there's always sections of the game where I have ridiculous lag spikes (going up to 500ms or more sometimes, which literally makes the game unplayable), which is frustrating during key sections of a match. These ping problems seem to have increased in frequency when I upgraded to 100mbps for free a while back (I can't remember when); the problem was never as significant as it is now.
I've tried playing with an ethernet cable; while the speeds are great, and the ping is slightly better, there are still frequent sudden ping spikes. As the router is in the living room, I don't think it's worth moving my laptop between the two rooms for a slightly better experience.
I'm sorry to read you are having issues with high pings whilst playing CS:GO.
We do advice to connect through a wired connection whilst gaming as you will experience better game-play. However, if you are noticing high pings through Ethernet cable also, are you able to post trace routes on both connections for us to take a look at please?