If you open the shader cache in the Steam settings (which is enabled by default) while using the Steam client, you will experience an abnormally long wait time when starting the game.

[Dracarcar]

I would like to report a bug to you: If you open the shader cache in the Steam settings (which is enabled by default) while using the Steam client, you will experience an abnormally long wait time when starting the game. Turning it off significantly speeds up the game. As far as I know, this is a widespread issue. Many people have encountered the same problem, which causes the game to start abnormally slowly, and it has been significantly improved after turning off this setting.

[Yujah]

What do you mean "open" the shader cache? Enabling Settings -> Downloads -> Enable Shader Pre-caching?

If so, I suppose Steam on start attempts to query new pre-compiled shaders being avaiulable or not, and something to be timing out for you as a matter e.g. of DNS-settings on your system. I.e., this to be a symptom of a general networking issue with your setup. Or in fact there to not be a problem but new pre-compiled shaders being indeed found available and getting downloaded first.

Both would suggest this not so much being a problem with that setting as such...