Last time I checked, there were no requests being made client-side in the 1-2 minutes it took to cancel. It was pretty much the same number of requests for both accepting and denying. Maybe they changed it since it's too blatant.
Also, since it should be opt-in, then accepting should obviously take longer.
Unlike GDPR, which uses a website as the gate for all cookies, the ad industry also has self-regulatory programs. Participation in these programs require that a website allow a user to opt out of all ad networks present on their site. TrustArc built a module to do that: https://preferences-mgr.truste.com/.
If you run the tool there, it will make a call to the ad networks listed. Of course if you're running an ad blocker, the call will get blocked and it will look like the tool doesn't do anything.
Unfortunately, many people simply click on the "accept all" button and don't care about their privacy that much.
The idea of GDPR was that consumers would be hesitant upon seeing the massive amount of third parties that use your data and demand change from the providers, turns out people don't care / providers rather let privacy-oriented customers suffer than to take a hit on their advertising profits.