Disable aero for csgo.exe. go to properties > compatibility > check "disable desktop composition".
Aero forces triple buffered vsync in WDM in borderless window mode which can act wonky especially on multi monitor setups. Probably what you're seeing now is even though your monitor is at 144hz and the game is reporting 300+ fps, the game window is being capped at a stuttery 60fps.
For games like csgo fullscreen is always preferred over borderless window since the extra hop in WDM always adds a bit of input latency.