FTS delays

[Joan Moreau via dovecot]

Hi 

When I do a FTS search (using Xapian plugin) in the BODY part, the
plugins returns the matching IDs within few milliseconds (as seen in the
log). 

However, roundcube (connected on dovecot) takes ages to show (headers
only vie IMAP) the few results (I tested with a matching requests of 9
emails) 

What could be the root cause ? 

Thank you