High CPU usage - GPU Process
I noticed that most of the time, Opera Stable 23.0.1522.77 uses too much CPU even on idle. My temps keeps skyrocketing over 100 degrees with high fan usage. I checked the Opera task manager and "GPU process:" keeps using the CPU even on just the startpage and not doing anything. Other times it's fine. Ending the process fixes it but when I restart the browser it's at it again. When checking Activity Monitor, Opera Helper uses more CPU than others. I haven't encountered this on Opera Next. That said, I still have not tested Next on Mac that much too see.
OS: OS X 10.9.4
Device: MBPr 13" late 2013
It seems the cause are the Speed Dial extensions namely Opera's Facebook and Twitter Notification extensions. Looks like they use a lot of CPU/GPU.
Running 10.6.7 and Opera 24 is just swallowing RAM - after an hour of usage, the app takes over 150 MB and there are 6-7 helper processes that take over 300 MB (one for each tab, I guess.) Gmail takes about 150MB per id/helper. With just 2GB, this makes using the browser difficult. bte, after reading your post, I deleted the fb and yt speed dial links and that did not help at all for me. There seems to be a bigger problem in the browser.
Fixed mine, it was a live wallpaper theme.