I have this issue with Opera consuming too much CPU, about 30% with a few tabs open and it easily hits 80-100% if I open more than 20 tabs, specially when their content is loading.
What I did so far was to set up the performance preferences in Windows Graphics settings, I added both opera.exe and laucher.exe to high performance.
Then I checked Opera's CPU report and got this error:
Disabled Features: gpu_compositing
GPU processing settings are all enabled as you can see in the images.
My set up is CPU: Intel Core i7-4790 4000 Mhz and GPU: Sapphire Radeon R7 250X, Windows 10 Pro x64
What else can I do to force Opera to use the GPU instead of cpu?