One of our client’s servers and sites has been running a bit slowly… and after a lot of research, changes to the site and server configuration, I found out why:
One person’s email inbox had accumulated over 57,000 emails in their inbox. Each time a new message was added, the server had to open up the entire mailbox, which took up a huge amount of memory and processor power.
So, if your server is running slow, take a look at the mail box sizes for all of the users (including forgotten admin ones). Cleaning them out might be the best way to speed up a slower server.