I ran a query on your wp_postmeta table and noticed that Beaver Builder is responsible for 625 MB out of the 850 MB (after clearing revisions) of the table size.
The _fl_builder_data (625.84 MB) contains every module, row, and setting used on a published page. It is stored as a serialized array (a complex string of data). When you edit a post, PHP must load the MB corresponding to that post string into memory to “unserialize” it, which is likely what caused you to hit the 256MB PHP limit previously, as it included the revisions and caused the crash.
Right now, this is only causing slowness as the thread “hangs” because it is doing massive amounts of work for just one user. This is why the site feels slow, even with plenty of available threads (32).
If one page has hundreds of Beaver Builder modules, that single row in wp_postmeta becomes a “heavy payload” that slows down every edit.