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Because of the dynamic nature of forums, its data can't usually be cached by normal caching plugins.  Combine that with the interpretative nature of PHP and you'll find shared hosting to not be adequate on large forums.  Very busy forums or large forums are going to need their own servers or own dedicated cores on VPS servers - a shared environment isn't going to cut it beyond a certain point.  After that you can get any WordPress performance expert to take a look at what your users are doing on the forum to make sure those database queries are optimized and that an object cache is in use.  (We optimize the queries for most use cases but what users do on certain forums might be different from "normal".).  Finally make sure you've got php 7.3 enabled.  7.3 is at least twice as fast as PHP 5.6.

Dynamic busy forums are altogether different animals - much different than an e-commerce store, your blog or your LMS. 

Oh, and all this might be affected by what your other plugins and themes are doing too.  So when optimizing, make sure those are all turned off and then turn those back on to see what effect they have on the forum.

I doubt that enabling and disabling the forum plugin is going to make a difference.

I have seen a few rare cases where rebuilding the database from scratch (i.e.: exporting the data from the WP database and importing it into a new database instance) made a difference to the WP site because of disk/data fragmentation.  But its been a couple of years since I've seen one of those cases - maybe because the new versions of mysql / linux are smarter about things like that??

Also, GTMetrix can be misleading and can send you down a path of spending days trying to optimize for milliseconds of gains.  What counts is how long it takes the first part of the page to show up for your users (regardless of what GTMetrix says.)  And many times that goes back to some of the things I mentioned above...