Explain MediaWiki configuration changes

README.md

@@ -172,6 +172,21 @@ WikiMedia foundation using it as a benchmark, and finding it fairly
 representative of wikipedia. A few other pages (HHVM, talk, edit) are also
 loaded to provide a slightly more rounded workload.
 
+- MySQL-based view counters have been disabled: they are incredibly inefficient,
+  unlikely to be used in any high-concurrency deployment (for example, Wikipedia
+  disables them), and will be removed in a future release of Mediawiki.
+- Localisation caching has been switched from MySQL to file-based. MySQL-based
+  L10N caching 'just works', but it is also inefficient (1 query per localizable
+  string per page view), and unlikely to be used in high-concurrency deployments.
+  This setting is also used by Wikipedia.
+- If `--apply-patches` is specified, the existing file cache is replaced with a
+  more efficient version that simply uses static PHP arrays. This has been
+  submitted to the upstream project at https://phabricator.wikimedia.org/T99740
+
+Each of these modifications improves performance under all tested runtimes: PHP5,
+PHP7, and HHVM. The concurrency issues with the MySQL-based view counters and
+L10N caching are contention within MySQL server.
+
 Profiling
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