Disable wp_mail plugin, why use that? Two smtp plugins at the same time?

Not my idea about email delay, it has been posted here very often.

You can maybe get some help using:

https://wordpress.org/plugins/gd-mail-queue/

a great piece of software.

Also enable debug log for a few hours, since your issue could be something else.

I tried "gd-mail-queue", and it seems to resolve the problem. I need more testing but I was able to post a new message that was sent in like 3 seconds instead of 15. 

I still use de WP MAIL SMTP PRO + i use gd-mail-queue.

@dimalifragis thanks for the idea!

1 topic has 50 follower (sends 50 email) : Post validation takes 15 seconds

1 topic has 12 follower (sends 12 email) : Pos validation takes 5 seconds...

So that might be my problem but no idear to solve it. I don't know if gd-mail-queue/ is compatible with WP SMTP EMAIL PRO that i use.

Not my idea about email delay, it has been posted here very often.

You can maybe get some help using:

https://wordpress.org/plugins/gd-mail-queue/

a great piece of software.

Also enable debug log for a few hours, since your issue could be something else.

@argentetsalaire OK, then the issue could be the outgoing email subscriptions. An email sending slowness. A slow smtp server or some related issue.

How do you send OUT the emails from wpForo?

An other thing to check:

Press F12 and watch your browser console for errors when you press to post a topic.

Each new post, i send about 50 emails notifications. I use WP Mail SMTP, witch uses the mail serveur that is on my CPANEL server. 

No consol error when i post a new post. 

But updating a old post is much faster. And when i update a post there are no email notification sent. So your idea about SMTP being the source of the problem is a good idea. 

Is there a way to send emails asynchronously, so that it doesn't penalize the creation of a new post?

https://wordpress.org/documentation/article/debugging-in-wordpress/

// Enable WP_DEBUG mode
define( 'WP_DEBUG', true );

// Enable Debug logging to the /wp-content/debug.log file
define( 'WP_DEBUG_LOG', true );

https://wordpress.org/plugins/query-monitor/

How do you send OUT the emails from wpForo?

An other thing to check:

Press F12 and watch your browser console for errors when you press to post a topic.

do you have the problem in every topic

or e.g. only in topic's with currently 1,851 posts and 75 pages?

NOW it is NOT caching. Disabled. Apparently you changed something and NOW it works right.

Is the performance issue solved?

I just add both 

/forum/
/forum/(.*)/

as @tutrix sugested. 

But no differences: The forum is still fast when reading posts, but still slow when publishing a new post. My host is providing a 12CPU and 48Go ram. I thaught, it could be faster. It takes 10 to 15 seconds to publish a new post.