[Dovecot] Resynchronization of indexes (or Warning: Maildir Scanning <dir> took <X> seconds)

Hello,

We have a dovecot servers farm with storage in NFS and indexes in local 

files. Under heavy load conditions we a log of messages in the form:

Sep 3 12:35:49 myotis32 dovecot: imap(<imapuser>): Warning: Maildir: Scanning <maildir cur directory> took 62 seconds (9516 readdir()s, 0 rename()s to cur/, why=0x80)

We have check that messages are directed with lmtp to the same server 

the user (scenario 3 at http://wiki2.dovecot.org/NFS with director in front of backend servers).

So my question is why is dovecot using these readdirs operations? why 

it is scanning the whole cur directory of the user? I guess this is an index resynchronization, but I don't know could it be the cause.

PS: I have attached my dovecot -n output