Opera 45.0.2552.812, latest Arch Linux.
Right from start Opera burns CPU with poll() loop as shown by strace: 
It is always 2 file descriptors, in this case, 110 and 117.
Looking at it closer, strace shows backtraces for each EAGAIN'ed recvmsg: 
Looks like it has something to do with X server interaction.
Checking those descriptors here: 
Closing those descriptors via gdb results in Opera SIGSEGV.
Always reproducible, even with clean profile and no extensions.
Is there any option how to save my CPU cycles?
Reproduced on 2 machines with Arch and on 1 machine with Gentoo. CPU usage varies depending on CPU power, but strace shows similar picture (EAGAIN flood).
Same applies to Chromium 58.0.3029.110. The only difference is that EAGAIN arrives more rarely (once per several, up to 10, seconds), thus CPU is not used that high as by Opera.
So, it has something to do with engine.
Same applies to 45.0.2552.881.
Same with 45.0.2552.888.
Having nothing to add to your problem, I would like to mention that chromium was updated to 59.x today from upstream. Check if that solves your problem.
Again, same applies to Chromium 59.0.3071.86, but to less extent. strace shows the same picture, but recvmsg fails with EAGAIN once per several seconds, thus CPU load is minimal. For Opera, recvmsg returns EAGAIN lots of times per second, and CPU load is noticeable.
So, this has something to do with common engine code, likely, interaction with X server.
Same with v45.0.2552.898.
Hi @pfactum, what you're seeing in strace is the message loop. Every UI application has one, and among other things, it handles communication with the X11 server. It is indeed very busy - a browser sends and receives lots of messages, many in one second. Getting EAGAIN is a normal part of life, and might indicate that Opera is busy (meaning that messages that are no longer relevant don't need to be handled). The EAGAIN is more likely to be a symptom than an effect of the high CPU usage you see.
If you still see high CPU usage while using Opera, check out the built-in Task Manager to see exactly which process is using CPU, perhaps then we can find out what's happening on your system.
@avl, in Task Manager, "Browser" process consumes 15 to 25% of CPU.
Still applies to v46.0.2597.26.
Still applies to v46.0.2597.32.
Same with v46.0.2597.39.