RSForm submissions too slow!—but easy to fix!

Joomla is a great platform for building web sites and RSForm is a great plugin for Joomla to handle form submissions—but with a recent project we found that RSForms was having some serious trouble with only a moderate amount of data (2,000 submissions). I tried to find the source of the problem and always got to the JOIN statement where it would routinely take 10+ seconds to run one query—add in multiple queries and filters and it would take over a minute just to get 500 or so records from MySQL. I almost gave up hope of finding a solution [...]