There is a process launched by launchd called efilogin-helper running as root, which is continuously running at high CPU load (60% to 120%)
Do you know what this process is and how I can get it under control?
There is surprisingly little information on Google.
Also, what would be a good debug method to go about solving these sorts of issues myself?
Go to system preferences => Desktop & Screen Saver
Select Apple->Desktop Pictures and choose the first option.
efilogin-helper will eventually quit after about a minute and no longer be a problem.
Bizarre but true!
(Once you have confirmed that you have indeed resolved the issue you can then probably reselect your preferred wallpaper without a problem.)