I have 100mb/s internet connection but im not getting nearly that.?

[Thomas]

About 1 month ago when I was downloading a game on steam I was getting 12mb/s. Now i'm only getting 2-3mb/s.

Same with my ping 1 month ago solid 20-30 ping in the game League of Legends, Now 60 - 5000 and often a solid 400 Ping all game.

I was wondering if it have something to do with my wireless adapter.

My Router: ASUS rt-ac66u
My Adapter: Jensen USB adapter (up to 150mb/s, 802.11n-standard ,WPS )

And when I use a online speed test i'm only getting 8-12mb/s internet connectivity. When I should get 100+mb/s (I've had it before.)
My Brother uses the same internet that I do and he dont have any problems with his ping.
I just think it's really weird.

(Im 10 feet away from the router only a door between the router and the adapter)