Currently testing before I go live. At the moment Im the only user on the forums so no traffic. I have WP-Rocket but its exceptioned for /forums so no issues. Performance in opening up and browsing most topics is pretty good except for very large topics with 1000+ posts in them.  I have a few topics with 2k-3K posts. Not a preponderance by any means, but lots in the 500-1000 reply range as well.

In opening these big topics to page 1, theres a significant lag time. For an example topic with 2k+ replies, the load time could be 25-30 seconds. If the post has 300-500 replies its 2-3 seconds which to me is acceptable.  I have my forums set to 25 posts per page to keep the size down.

In querying the MySQL on these mammoth topics, where is the heavy lifting being done? Is this where hardware/hosting specs really come into play in terms of CPU and MEM, or is it something else php.ini or?  My host is on a small plan at the moment (1vCPU, 1.5GB mem) just to get it up and running but I plan to go VPS with 4-8 vCPUs and 8-12 GB mem -- is this the likely bottleneck or something else software/WP/WPForo side?

Once the massive topic is finally opened to page 1, tabbing to page 2, page 3, etc is quicker. 

If I open up a topic with 200 replies its no real issue (lets see however once I have users on pounding the site).

Any thoughts or input appreciated. I'm hoping the answer is "yeah those are the specific instances where you need horsepower" because those wheels are already in motion.