This will happen to some people, who very like to try a lot of plugins. Then, some of the outdated plugins or bugs maybe cause the problem. 

It is simple to solve the problem. Just turn off the plugins one by one to check if the vscode speed up. But this is very cumbsome, the simplest way is: 

press F1, then input “show running extensions” , or just click here:

then,  disable the plugins which cost a lot of time to start up. It is also important to keep in mind, don’t keep too many unsed plugins in running. It will also slow the the vscode speed.