I occasionally get lag or a stutter on videos when using GX. I don't have this issue with Chrome, however. What I've noticed is that if you watch Youtube videos with anything major running in the background (usually RuneScape for me), that's when this happens and I can usually fix it by restarting the browser.
However, I did find a thread on the main Opera forum that addresses this issue and it seemed to help me, so maybe it'll help you too?
Also, regarding Opera's built-in ad blocker, I use standard Opera on my laptop and I just run the built-in blocker (for the sake of performance) and I notice that it doesn't block as many ads as I wish it would. I use GX on my PC and I ended up installing my favorite ad blocker (Adguard) on it because it always worked exceptionally well on previous browsers. Thankfully, it works just as well on GX.
