Manual:$wgInvalidateCacheOnLocalSettingsChange

Setting this to true will invalidate all cached pages whenever LocalSettings.php is changed. This is useful so that if you install a new extension, any old pages that were cached before the extension was installed are re-rendered.

On high-traffic wikis, this should be set to false, to avoid the need to check the file modification time, and to avoid the performance impact of unnecessary cache invalidations.