Plesk Panel itself works fast out of the box and sites (if they don't have code issues and if the server has enough resources) also work fast.
Plesk will automatically increase Apache and Nginx "open files" limits in case you create many domains.
One thing is that Plesk doesn't configure Mysql/Mariadb which means that default settings are used.
You can optimize Mysql/Mariadb performance using MySQL tuner from https://github.com/major/MySQLTuner-perl
Please note that this task is not related to Plesk Panel, It’s a server-level work.
General recommendations for Plesk Panel optimisation are :-
TIP # 1. Install Nginx
For Installing end enabling Nginx reverse proxy via the Plesk interface, Follow the mentioned steps:-
Step 1. In Plesk, go to Tools & Settings and click Updates and Upgrades.
Step 2. On the Updates and Upgrades page, click Add/Remove Components.
Step 3. On the Add/Remove Components page, expand Web hosting and select the Nginx web server for installation. Click Continue to begin the installation.
Once the component is installed, enable the Nginx reverse proxy service in Plesk :
Step 4. Go to Tools & Settings > Services Management.
Step 5. Click the button next to Reverse Proxy Server (Nginx) once and wait. The process will take some time to configure.
TIP # 2. Use highly possible PHP version for sites. It's better to use php-fpm handler instead of fastcgi.
TIP # 3. Do not install extensions from Plesk > Extensions that will not be used.
TIP # 4. Always update your Plesk Panel and it’s server components for better performance.
