Why, after adding certificates to the browser, does it start to slow down?

Question

Browsers

I had to add 10 certificates to browsers (Chrome and Firefox). This was needed for one online identification system.
Purely visual - after that, the opening of the pages began to slow down, and in both browsers.
Why can this be? Maybe the browser is trying to somehow check the validity of the certificates? I have no idea what's going on under the hood.

Answer 1

Of course it tries, if they are not root. He contacts the publishers' servers, asks them for a CRL (if there is a link to it, and the purchased ones usually have it, it can still compare timestamps)

Answer 2

I had to add 10 certificates to browsers (Chrome

Chrome takes certificates from the system.