Some pages are loading slow, other fast | some actions (add discussion...) are slow

The issue or problem
Two weeks ago, suddenly, on two instances of OJS (on different domains on the same Linux Debian 10, PHP7.2, MariaDB 10, OJS some pages started to load very slowly (time to first byte received by the browser request 7 seconds), for example: the Home page, the page of any Issue, the page of any article, and in the Back-end: Journal settings page, Administration.
Other pages, are loading OK (time to first byte received by the browser request 0.9 seconds): Archives page, Announcements, Submission, About, and in the Back-end: Dashboard, Website, Workflow, Users

Furthermore, when clicking buttons like: after opening a discussion, the OK button to send the discussion takes 12 seconds after being clicked, with the rotating image, to get to finalize, or Register button after entering the registration information. The action are finished successfully, but as observed, veeery slowly.

Steps I took to try to solve the issue
First, I cleaned the cache from the Administration, than manually in the cache folders. No results.
Then I run Optimize Table on my SQL tables. No results.
Restart mysql service and apache2 service, and then even the server. No results.

Next, I thought it might be a virus. I compared with Meld (a visual folder/file comparison) of the web folder from present and one one year old. The differences were only the lines of code that I changed myself.)

Finally, I duplicated one instance of OJS in another folder, and upgrade it to 3.3.0-14. I installed for it a php7.4. I set as default in /etc/mysql/my.cnf the InnoDB database engine. I modified all the tables before upgrading to InnoDB database engine. The upgrade succeeded. But, the same pages, and buttons remain slow, as well as buttons.

The web-server is running on a Dell PowerEdge R440 Intel Xeon Silver 4110 Cpu @ 2.1 Ghz, 8 core, 2 threads per core, 16 Gb DDR4 RAM, 250 GB SSD hard disk.

Any help would be great!