Re RAM cache - Moodle is written in PHP, which unfortunately means that caching information in RAM between requests is next-to-impossible; there are some horrible hacks (hello, memcache) which can enable it, but these generally require extra software installation and maintenance.
In many parts of Moodle, possibly in quiz too, data is cached in user sessions to save repeating complex queries. Normally these are not cached in RAM exactly but on disk (still fast, probably ends up in RAM through disk caches) or database (slow but at least it's one single simple query, albeit with large data). Theoretically you can also put them in memcache if you like.
Really somebody would need to profile quiz to find out exactly which functions/queries are taking longest to process a typical request, I don't know if Tim has done that with his new question engine. If we have a profile, it might be possible to work out whether specific things could be cached to improve performance, but great care would need to be taken not to reduce reliability; after all you can usually solve performance problems as simply as getting a new server, but reliability problems (in a tool used for formal assessment) are much more critical.
Just my opinion anyhow.