mdworker process always at 100%, even when not indexing
Ok it seems to be fixed now. If it can help someone else, here's how to debug the issue:


Open the "Activity Monitor" application.
Click on the "mdworker" process.
Click on the "Inspect" button.
In there, click on "Open Files and Ports"
Somewhere in this window, check which files are open. Among others, look for the ".mdimporter" files (Spotlight plugins).
If one or more mdimporter file always seems to be open, you can disable it by renaming to something else. Open the terminal and run sudo mv /path/to/the/file.mdimporter /path/to/the/file.mdimporter.disabled
Then reboot your computer.


In my case, it was the file /Applications/Britannica 13.0/Ultimate Reference Suite.app/Contents/Library/Spotlight/BritannicaBookmark.mdimporter that was constantly open. After renaming it, things are back to normal.