@vybo Not more than once the process is known, it might give a better idea of what's causing the load.
If it's an extension process or a tab process, the cause would be obvious (heavy site, bad extension etc).
If it's the GPU process, could be a GPU driver issue.
I believe any CPU load from Mail/RSS/Calendar client would come from the Vivaldi App process and also the Browser process.
If it's the Browser process it's basically Chromium code so a lot harder to know what is going on.
And so on... this isn't an exact science, it's a lot of guesswork for us that don't know Chromium's code 😆
Of course, if one is advanced, you could use Process Explorer and view the process' threads, might get closer to the answer. If one adds the Windows and Chromium symbols files to ProcExp it might even give the current method call being done in the thread.
Next step would be to attach a debugger and maybe generate some native dumps to examine the data structures in the process. This is what a developer would do if they saw the same problem on their machine.
I'm not a Chromium developer nor do I work at Vivaldi, so I'm stuck at the "mostly guessing" part 😉