I usually recommend Brightness Controller (Which I reference in the following thread) for a GUI app for handling monitor brightness. Contrast, however, I am not so sure about.
Please review this thread:
I did try out DisplayCal since if I suggest an app, I will sometimes install it in order to know that it will easily install for the user and is functional. I liked it a lot, actually and it has a LOT of options.
I think there may be a typo in this sentence; I may be misunderstanding what you are looking for.
Many Browsers have a setting within the browser to enable or disable Hardware Acceleration.
It may seem intuitive to enable it - But I suggest the opposite. Instead, disable hardware acceleration and then test the Youtube performance with that option disabled.
EDIT: Correction; I do not reference Brightness controller in the other thread (I thought I did... I was wrong). Here is how to install that app:
sudo add-apt-repository ppa:apandada1/brightness-controller
sudo apt update && sudo apt install brightness-controller