Usually, a webpage response is equal to Document Complete + up to 2 seconds. Can it go higher in any case? if Yes, when? Can I get some information on this please?
It is after the last network activity after document complete, so yes - it can easily be a LOT higher. The 2 seconds is the amount of time that it needs for there to be no network activity but that isn’t included in the time.
The +2 seconds isn’t a hard and fast rule and depending on what your website is doing, could be extended. For example, if you have a website that has an animated slider, a developer may program your website to load the first 2 slides in the initial page load and defer the loading of the others until after the page has loaded - in which case, this extra resources will trigger additional network activity and effective “reset” the 2 seconds.