I created a new forum and a new post topic but it's still slow. Posting is still around 4 to 5 seconds. All wpForo cache options are enabled.
We just finished the checking and found the main problem maker.
1. 90% comes from jReviews plugin. This plugin generates the most problemful error in WordPress. it stops WordPress loopback and keeps submitted processes loading until the 10001 ms timeout.
The loopback request to your site failed, this may prevent WP_Cron from ...
Error encountered: (0) Connection timed out after 10001 milliseconds.
This error can be found in WordPress website Health Checker ( Dashboard > Tools > Health Checke).
This error affects all processes not only wpForo. Once we deactivated this plugin, the Health Checke stops reporting the problem. And the topic/post submitting becomes much faster.
2. The second is wpForo MyISAM tables. After the migration the tables with _wpforo_ prefix were not changed to InnoDB. but this is just a 10% speed affection, not more.