Redirect root to folder

have you read Avoid landing page redirects | GTmetrix ? you are doing exactly what they said not to do = on landing page doing a redirect. The solution is moving /folder site to web root of site at / if that is the main landing page content you want to serve when someone accesses /

 

Then just advertise the forum url not main domain. As the article says, if you need to do a redirect then do it. It doesn't say not to do it ever

I prefer subdomain rather than folder myself. It allows me to host forums on separate server from main site i.e. centminmod.com vs community.centminmod.com to only scale one server/site when needed.

 

Yes in letsencrypt and centmin mod if separate nginx vhost you get separate letsencrypt ssl certs one for domain.com and one for forum.domain.com. But for my centminmod.com sites and subdomain, I use Comodo paid SSL wildcard SSL cert so doesn't matter as I am covered for *.centminmod.com

 

I have 100s of subdomains off of my domains, so SSL wildcard is easier to use and allows me to spin up subdomains at will all on the cost of a single SSL wildcard and since I also use load balanced server clusters, can provision SSL wildcard easier as Letsencrypt needs to validate each domain to a specific server IP.

 

similar process to Wordpress - centmin.sh menu option 22 - How to switch Wordpress to subdirectory just in reverse