One of the major positives of having a VPS server is you can make these type of changes to it so you should take advantage of that opportunity if you have a VPS server.
1. Disable Mailman
What is it? Mailman is a free software for managing electronic mail discussion and e-newsletter lists. If you have Wordpress you don't need this running too as there are plenty of Wordpress plugins that can help manager e-newsletter lists. Also there is PHPList available which only uses resources when it needs to.
Why? Mailman sucks up memory constantly for no reason
How? WHM >> Server Configuration >> Tweak Settings >> Turn it off by unchecking the box.
2. Adjust Number of Minutes between Mail Server Queue Runs
What is it? Number of minutes between mail server queue runs. Smaller number means it runs more often.
Why? Each time it runs it increases load on the server (aka it sucks resources)
How? WHM >> Server Configuration >> Tweak Settings >> Mail >> Default is 60 - change to 120 or 180
3. Reduce Memory Strain From SpamAssassin
What is it? SpamAssassin is a computer program that uses a variety of spam-detection techniques and tries to reduce the amount of spam you receive.
Why? Even though SpamAssassin sounds goods it very well can slow down your system too. Simple tweak will reduce that load.
How? In WHM search for "Manage Plugins" or scroll down until you see it. Click on Manage Plugins then you'll see spamdconfig - enable that. Once done refresh the frame in your browser (or wait a minute - log out of WHM then log back in). Now you can search for "Setup Spamd Startup Configuration". You'll have 4 option boxes you want to pay attention to the 2nd one. Set "Maximum Children" to 2 which will restrict the number of processes SpamAssassian fires up on startup. Save this and give the server a minute to refresh.
There's 3 simple steps to speed up Wordpress on a VPS server. In just about 5 minutes you have saved your server's memory and processing power and potentially sped up your websites!
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