Yeah, a few of the recent patches on both the Windows side, and the CSGO side have decreased performance for a lot of people. There are a few fixes for it, just google it and you'll see plenty of examples.
There's a lot of conflicting opinions on whether or not there is a difference between larger FPS numbers with CSGO, some people say there's no difference, some say there's a huge difference. Here's my two cents.
I've been playing CS for years, I played both Source and Go shortly after they came out, and have been playing them consistently ever since. I played CSGO semi-professionally on a sponsored team for a few years, and I've clocked many hours in game. I've played on a lot of different setups and here's what I think of the changes in FPS:
Obviously any changes between 0 and 60 fps are going to feel very different. For example, 60 feels way better than 40, and 40 feels way better than 20. This is a given. As for 60+, many say that beyond 60 you will see no difference with a 60hz monitor. I don't find this to be true. I've found that for a 60hz monitor, there is no difference past something like 120 fps. 60 fps feels smooth, but I definitely play better with an unlocked framerate, and I've found that the difference stops at 120 fps give or take.
Obviously if you're running 144hz+, the changes vary with the refresh rate. You'll feel big differences all the way up to 144 fps, and little differences out to around 160-170 fps. Not saying any of this is proven, this is just what I've found to be true as far as my experience with the game goes.