Confirmed Template list query is extremely slow on large installations

[Painbaker]

Affected version

2.3.13

The ACP template list query never uses the existing style_id_type_title making the page slow to load:

SELECT `xf_template_map`.*, `xf_template_Template_1`.*, `xf_addon_AddOn_2`.*
FROM `xf_template_map`
INNER JOIN `xf_template` AS `xf_template_Template_1` ON (`xf_template_Template_1`.`template_id` = `xf_template_map`.`template_id`)
LEFT JOIN `xf_addon` AS `xf_addon_AddOn_2` ON (`xf_addon_AddOn_2`.`addon_id` = `xf_template_Template_1`.`addon_id`)
WHERE (`xf_template_map`.`style_id` = 1) AND (`xf_template_map`.`type` = 'public')
ORDER BY CONVERT (`xf_template_map`.`title` USING utf8mb4) ASC
LIMIT 300

Run Time: 2.917213
Select Type    Table    Type    Possible Keys    Key    Key Len    Ref    Rows    Extra
SIMPLE    xf_template_Template_1    ALL    PRIMARY                   3890    Using temporary; Using filesort
SIMPLE    xf_template_map    ref|filter    style_id_type_title,template_id,type_title    template_id|style_id_type_title    4|26    test.xf_template_Template_1.template_id    1 (47%)    Using where; Using rowid filter
SIMPLE    xf_addon_AddOn_2    eq_ref    PRIMARY    PRIMARY    52    test.xf_template_Template_1.addon_id    1    

    XF\Db\Mysqli\Statement->execute() in src/XF/Db/AbstractAdapter.php at line 96
    XF\Db\AbstractAdapter->query() in src/XF/Mvc/Entity/Finder.php at line 1461
    XF\Mvc\Entity\Finder->fetch() in src/XF/Admin/Controller/StyleController.php at line 196
    XF\Admin\Controller\StyleController->actionTemplates() in src/XF/Mvc/Dispatcher.php at line 364
    XF\Mvc\Dispatcher->dispatchClass() in src/XF/Mvc/Dispatcher.php at line 266
    XF\Mvc\Dispatcher->dispatchFromMatch() in src/XF/Mvc/Dispatcher.php at line 121
    XF\Mvc\Dispatcher->dispatchLoop() in src/XF/Mvc/Dispatcher.php at line 63
    XF\Mvc\Dispatcher->run() in src/XF/App.php at line 2840
    XF\App->run() in src/XF.php at line 850
    XF::runApp() in admin.php at line 15

Suggested fix: Force the finder to use style_id_type_title