Recently noticed this message in the activity log, which unfortunately dates back to November 2016, probably during an upgrade from 7.2.2. Previewing, opening and importing downloaded email takes 1-2 minutes each, all other SuiteCRM activities are satisfactory.
Saw the suggestion at https://github.com/salesagility/SuiteCRM/issues/1007
Recommends to create the following empty files in modules/AOD_Index/Index/Index/
Created the following files (with the exception of (read.lock.file that already existed):
-rw-rw-rw- 1 www-data www-data 0 Feb 27 03:00 optimization.lock.file
-rw-rw-rw- 1 www-data www-data 0 Feb 27 15:01 read.lock.file
-rw-rw-rw- 1 www-data www-data 0 Feb 27 03:00 read-lock-processing.lock.file
-rw-rw-rw- 1 root root 43 Feb 27 03:00 segments_1uc
-rwxrwxr-x 1 www-data www-data 20 Feb 27 03:00 segments.gen*
-rw-rw-rw- 1 www-data www-data 0 Feb 27 03:00 write.lock.file
This did not resolve the error message. It’s possible that there is another /index/(contents) that is needed after the upgrade but without a reference point I think it would be dangerous to create them.