Aside from a slow internet connection, there are other things that could be causing high ping, such as your PC settings, the server youre choosing to play CS:GO on, or bandwidth hogging applications. An old network adapter, less storage, and memory along with a GPU can all lead to high ping and lag issues.

Why do I have such high ping in CSGO?

Quite often high ping or lag will be due to a temporary network problem related to the routing of network traffic between you > your internet provider > our server provider. If you started the game via the launch button, try to connect to the server by using the IP address instead.