WordPress load all the resources on every page load.
So it take about 20Mb of memory to generate each page.
The easiest way to speed up WordPress is to exclude not needed files.
Open /wp-settings.php file and comment lines which contain these strings:
- deprecated.php - contains old functions
- pluggable-deprecated.php - contains old functions
- bookmark.php - contains bookmarks functionality which majority of users do not use
- bookmark-template.php - contains bookmarks functionality which majority of users do not use
Commenting these lines will help you to save few Megabytes of memory on every page load. The pages will be loaded faster and you will save some server resources.
If at some point WordPress can throw an error because some outdated plugins or themes are still using code from these files. To fix this you need to uncomment these lines back.
And also this file can be overwritten after WordPress update and you need to comment these lines again.