SRD75

Once the Memory Usage Monitor plugin is activated, activating or deactivating any other plugin will display the memory used by that plugin. e.g:
Memory usage for WP Memory: 0.02 MB
For accurate memory usage breakdown for each plugin, you must activate or deactivate them individually; you will not see memory usage when bulk activating plugins.

Memory Usage Monitor

Displays memory/RAM usage for each plugin as they are activated and deactivated.

nikdow

Use this plugin to display detailed information about your server’s OPCache, including memory utilisation, list of all scripts cached and a useful memory utilisation visualisation based on the file system tree.
This plugin is based on https://github.com/rlerdorf/opcache-status which is for general PHP use, we have turned it into a WordPress plugin for ease of installation. Apart from some rearrangement of scripts to suit WP load conventions, the code is to date unaltered. WordPress capability ‘activate_plugins’ is required to see it in the admin menu and to run it.
After you install and activate this plugin, look in the admin Tools menu for OPCache.

OPCache Scripts

Use this plugin to display detailed information about your server’s OPCache, including memory utilisation, and a list of all scripts cached.

Memory Usage Monitor vs OPCache Scripts