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I ran EtreCheck because of performance issues, and it tells me that mdworker(13) is the “top process by CPU” with 56%. Is this normal, or should I do something about it? Mdworker is a background daemon and is responsible for compiling the metadata indexes that are used for Spotlight searches. There are usually several copies of it running at any time, and each should typically take less than 5% of CPU time, though sometimes when working on large files they can exceed that — but never as much as 56%.
High CPU usage by mdworker normally occurs when it’s choking on a file, causing it to crash repeatedly and be restarted automatically. This is normally recorded in the log, and should be easy to spot in Console (which you’ll find in /Applications/Utilities) or Consolation (free from eclecticlight.co). Log entries normally record the full path and name of the file that is causing the problem.
One common cause for this is that the file it’s trying to index has become damaged — for example, one of its extended attributes has become corrupt. You can try First Aid in Disk Utility, although that often doesn’t find a problem.
Ultimately, you might need to delete that file, or restore it from your Time Machine backup if it’s important. Sometimes, after a restart or two, you’ll find that everything settles down spontaneously.