Happening to me as well, chrome users have reported this since feb 2018, so it annoy us as well. Hoppefully they will fix it.

For now my fix is copy and paste the google link you want to enter(yeah all google services are somehow "softlocked" for me and not reachable from time to time) in internet explorer(yeah.) and then refresh the page in opera. It works for me idk why.

Opera 51
Windows 10 fall update
8gb of ram
ryzen 1600
gtx 1070

With 2 tabs (html5 video, this page) : 850mb

The max I've seen was 2.5gb (10 random tabs open)
With normal use is 2gb on avg anyways(3 tabs & 2hours open)

There is possibly a ram exploit but has to be reproduced with certain activities such as: leaving video open in the background, having both messenger and whatsapp in the background

So for 4gb users can be big hard time. Possible fixes? Closing the app and opening it again. Killing messenger and whatsapp. Disable extensions

For developers there should be some kind of ram opmitization, what Firefox quantium does is that only prioritizes the tab that is open at the moment. Reducing a lot of cpu and ram usage

Hope this helps