If you maintain a very large number of files on your website, it can have a substantial negative effect on performance and stability, especially if they are all contained in the same directory. If your site requires a large number of files, maintain them in multiple directories;
Using standard Views pager might cause performance issues because of additional COUNT query. Use Views Litepager instead.
Some conditions prevent the use of an in-memory temporary table, in which case the server uses an on-disk table instead:
Presence of a BLOB or TEXT column in the table;
Presence of any string column in a GROUP BY or DISTINCT clause larger than 512 bytes;
Presence of any string column with a maximum length larger than 512 (bytes for binary strings, characters for nonbinary strings) in the SELECTlist, if UNION or UNION ALL is used.
Drupal Website Launch Checklist (the best practices)