It’s a bit difficult to diagnose the cause of this type of problem just from the symptom (slowness), without access to the server. So I’ll comment on some possibilities that I’ve seen occur in other OJS.
Apparently the bottleneck is associated with the database server.
Do you have thousands of users registered in the database of this OJS? In particular, those users who we’ve noticed don’t look like real people (e.g. Collaborative list of spam user patterns ). Depending on the number of users and the computer resources available, this can negatively affect the performance of the OJS.
Check whether the “Most read articles by the same author(s)” plugin used in 3 journals may be affecting the performance of this OJS. For example, try disabling it in all of them and see if performance improves.
Check the server access logs to see if there are many bots indexing the journal and if there are any of them accessing the journal at very short intervals.
If the database used is MySQL/MariaDB, check that all the tables use Innodb.
Does your OJS use shared hosting? If so, it’s likely that your provider adds restrictions beyond memory and CPU usage. We, for example, host the OJS on a VPS server for better visibility and to avoid headaches with the restrictions mentioned.
The delay in publishing an issue may be associated with sending an e-mail notifying users. The number of users and the time it takes to send each message in your current configurations count for a lot. Newer versions of OJS send messages in the background and will probably provide a much better experience.
I hope this message can help you.