"skiplist: longlock" on conversations DB

Hello.

On a server of mine (running 3.4.9) I'm often seeing the following in 
the logs:
> lmtpunix[34158]: skiplist: longlock /var/imap/user/c/xxxxxx.conversations for 5.4 seconds

xxxxxx is always the same user, which, so far, has not complained.
Her conversation file is 144 bytes long, like all the others (at least 
right now).

What does this message mean exactly?
Should I worry or just ignore it?

Thanks in advance.

  bye
        av.

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