Major room for performance improvement in filter_glossary (backport of MDL-47962)

Copied and pasted from MDL-47962 : The main thing to test here is that there are no regressions in relation to the glossary filter, so redoing MDLQA-11715 and MDLQA-11716 would be a good test. It would also be good to verify that there are not regressions in other filters that use the filter_phrases function (Activity names, Censorship and Database filters). As far as I can see (e.g. tracker queries project = MDLQA AND text ~ 'filter' AND component = "Database activity module" AND parent = MDLQA-11698 or project = MDLQA AND component = "Filters" AND parent = MDLQA-11698 there are no manual tests for this, which I take to mean that the automated tests for these are sufficient. Finally, you could either look at my test results below, and consider them sufficient, or replicate them yourself. You would need to: Create a course with many lables (or many activities set to show description on the course page). Create a main glossary containing may terms. (An import from qa.moodle.net is a good way to get this.) Ensure the glossary filter is turned on. Turn on display of performance info in the footer (or turn on profiling) Verify that without this fix the course page loads slowly, and with it it loads fast.