If you profiled further, perhaps a lot of what was happening was frequently the same sort of processing? Be that building up the html page to serve or querying the database. Did you have Redis or memcache as cache layer? It'd use RAM as an in-memory key store(DB) to cache responses you'd serve for requests to the server. That'd help reduce CPU load. For PHP, using PHP-FPM might have helped quite a bit, as well as upgrading to newer PHP versions (I migrated MWS from PHP-5 to 7 in 2016, from their prior hosting provider to a better suited one, they had user load issues too and the migration was quite the improvement in perf as well as cost, at least for not actually changing any of the actual code).

As for diskspace, that's an interesting one? Weren't you largely just working with text data? If you stored structured text(like JSON), that shouldn't use up that much, you could also benefit from compression much more in such a format.