Mit dem nachfolgenden Apple-Script kann man Mail unter OSX 10.11 wieder beschleunigen.

Das Script beendet Mail, entstaubt den SQLLite Index und öffnet Mail wieder.
gerade bei sehr großen Datenbanken kann das zu einem deutischen Stabilitäts- und Geschwindigkeitszuwachs führen.

(*
Speed up Mail.app by vacuuming the Envelope Index
Code from: http://www.hawkwings.net/2007/03/03/scripts-to-automate-the-mailapp-envelope-speed-trick/
Originally by "pmbuko" with modifications by Romulo
Updated by Brett Terpstra 2012
Updated by Mathias Törnblom 2015 to support V3 in El Capitan and still keep backwards compability
*)

tell application "Mail" to quit
set os_version to do shell script "sw_vers -productVersion"
set mail_version to "V2"
considering numeric strings
    if "10.10" <= os_version then set mail_version to "V3"
end considering

set sizeBefore to do shell script "ls -lnah ~/Library/Mail/" & mail_version & "/MailData | grep -E 'Envelope Index$' | awk {'print $5'}"
do shell script "/usr/bin/sqlite3 ~/Library/Mail/" & mail_version & "/MailData/Envelope\\ Index vacuum"

set sizeAfter to do shell script "ls -lnah ~/Library/Mail/" & mail_version & "/MailData | grep -E 'Envelope Index$' | awk {'print $5'}"

display dialog ("Mail index before: " & sizeBefore & return & "Mail index after: " & sizeAfter & return & return & "Enjoy the new speed!")

tell application "Mail" to activate

Quelle