Optimal Frames Per Second
So as to calculate the optimal number of the FPS, we need to multiply the refresh rate of your monitor by 2. Consequently, a 60 Hz monitor would need 120 frames per second, while a 144 Hz one would require about 300. The idea of multiplying the value is based on the way the frames are rendered.
With the vertical synchronization off, the monitor will always draw the newest frame, which means 120 frames per second on a 60 Hz monitor will get you the smoothest picture possible for such a monitor. Further increases in the FPS will make sure you don't notice any frame drops.
Resultion and Settings
Lowering the resolution is the most obvious and efficient way to boost the FPS. This method helps if your graphics card cannot handle higher resolutions. Moreover, many pro players recommend lowering the resolution to make the game more comfortable.
As for the settings, the most resource-demanding ones are the quality of shadows, shaders and effects. The "high" shadows setting might help in certain situations. However, it doesn't help that often, which makes it a questionable decision to switch it on. It could lead to possible FPS drops that would be constantly annoying you during your game.
Other settings such as "Mode / Texture detail" or "Texture filtering" should be lowered as a last resort. Most modern PC's should be able to handle those with no issues.
Drivers and Game Mode
Your graphics card drivers should be regularly updated since they might boost the FPS a little. You should keep the other hardware up-to-date, as well.
Windows 10 users might try to boost their FPS by turning on the so-called Game Mode. All you need to do is launch CS:GO, tap Win+G and select the "Use Game Mode for this game" option in the General tab of the appeared window.
Graphics card settings
The process is similar for both AMD and Nvidia graphics cards. First of all, choose the "maximum performance" setting in the "Adjust image settings with preview" tab. Then, go "Manage 3D settings" and change all the values related to antialiasing or filtering to "performance" or "off". You can also try changing the "Maximum pre-rendered frames" value to 2 or 3, but that might not change anything.
The AMD software has no such slider, so we can proceed to the "Gaming settings" tab right away where you need to repeat the same steps as for Nvidia. One setting is worth mentioning, though, it's preferred to leave "Shader cache" enabled.