ways to improve your server speed without the need to increase server's memory or CPU

There are many ways to improve your server speed without the need to increase server's memory or CPU. If it relate to your website, you may consider to install caching. Such as for Wordpress, there is guideline how to utilize caching. Refer following link:

https://codex.wordpress.org/WordPress_Optimization/Caching

Others, optimize your site's scripting to lessen make external call, use compression and so on. Refer following link for other tips:

https://www.crazyegg.com/blog/speed-up-your-website/

https://bobcares.com/blog/how-to-increase-your-server-speed/

A website giving lots of errors may give impact to your browsing speed too, though in browser the website appear okay. Lots of queued email may also one of reason why your server become slow. This maybe either your server being spam or one of your mail user been compromised and their mail account been used to authenticate for sending spam emails.