Peak Memory Use Varies for Same Queries

This likely means that something other than a database query is causing the memory usage to fluctuate, for example an expired cached item that gets regenerated during the page load, or an HTTP API request that sends a request (you can see this in the HTTP API tab). This sort of PHP processing isn’t easy to identify so you’ll need to do some digging.

See https://github.com/johnbillion/query-monitor#related-tools for some related tools that might be useful.

Much appreciated. I’ll look into this, see if I can track it down.

There is 2 sites on my server one is always slow and one is always fast and the last site to receive an update is always the fast one !

I’ve found the reason. Opcache memory was full and so was only working for 1 site at a time.

Using the plugin WP OPcache I can see what opcache is doing.

Then for my server I edit the opcache config file which in my case is
sudo nano /etc/php/8.1/cli/conf.d/10-opcache.ini

I added these settings:
opcache.memory_consumption=512
opcache.interned_strings_buffer=64
opcache.max_accelerated_files=32500
opcache.revalidate_freq=60
opcache.enable_cli=1
opcache.enable=1

restart php which in my case is
sudo service php8.1-fpm restart

and both sites now work the same.