Only use SSL certificates for sections and services where they are truly needed. This is because encrypting all of the HTTP data stream requires a large number of CPU power. And so encrypting a page’s static elements such as images (except the content) uses up considerable resources while presenting no benefits.

SQL queries inside a loop seriously hinder performance, especially if the SQL server(s) are not on the local machine. That is because these servers are designed to handle several selections, insertions or modifications in one single query or transaction.
Wrongly used, these queries unnecessarily consume CPU power, RAM and bandwidth.