Add performance enhancements for WP

[roborourke]

Core WP has some performance issues that may only reveal themselves in certain situations. We can start adding some simple mitigations in this module:

• force limit comments per page to 50 maximum
• add https://github.com/Automattic/advanced-post-cache
• add https://github.com/Automattic/lightweight-term-count-update

ht/ @johnbillion

Acceptance criteria:

• Comment pagination is always enabled
• Impossible set comments per page to a value higher than 50
• Advanced post cache is required in composer,json
• Advanced post cache file is loaded on the muplugins_loaded hook in inc/namespace.php
• Lightweight term count update is required in composer.json
• Lightweight term count is loaded on the muplugins_loaded hook in inc/namespace.php
• Above plugins are added to the exclude list in humanmade/altis-core here https://github.com/humanmade/altis-core/blob/master/inc/composer/class-installer.php#L26
• Review and document performance tweaks from https://github.com/Automattic/vip-go-mu-plugins-built/tree/master/performance here to see what else we could add

[johnbillion]

Background:

• Advanced Post Cache caches post queries in regular loops. I've not tested it heavily but it does remove the main post query once the cache is warm.
• Lightweight Term Count Update removes the slow queries that core performs when recalculating the number of posts assigned to terms after updating a post's terms. On the RS DMS project with ~6M rows in the posts table these queries take 20 full seconds for each taxonomy. Notably, this isn't enabled during cron and CLI requests, which is where we were seeing terrible performance problems recently. We enabled it for cron events according to Allow new counting in CRON Automattic/lightweight-term-count-update#9.

In addition:

• VIP implements a meta_key+meta_value index on the postmeta table. The index can get
  huge but it does allow for performant meta_value queries, and we're actively using them on RS DMS. https://github.com/Automattic/vip-go-mu-plugins-built/blob/97ee6f2abc79bd1bb5e9149f5d63e5f201d179be/schema.php . Joe H indicated he'd be hesitant about implementing this as-is, but it's certainly something to consider if for some reason ES isn't a viable solution for a particular site.

[johnbillion]

VIP has some more performance tweaks here but I've not looked at them in depth: https://github.com/Automattic/vip-go-mu-plugins-built/tree/master/performance

[dashaluna]

@roborourke could you point where to add comment limitations please?

[johnbillion]

We enabled it for cron events according to Automattic/lightweight-term-count-update#9.

Relevant link for enabling LTCU during cron events: https://github.com/humanmade/rs-dms/blob/054cc7468403c4efe97f1793b46e6a5d26bbc622/client-mu-plugins/rs-bulk-management/inc/functions.php#L68-L72

[dashaluna]

Both packages mentioned in the description aren't on packagist.

@rmccue is checking if there are forks/substitutes to use instead.

[roborourke]

@dashaluna the comment pagination code should go in performance namespace, you should find it under the inc directory

[roborourke]

@dashaluna sorry! My fault, the code I was thinking of is in the cloud module. In this case you can add the code to the inc/namespace.php file.

[dashaluna]

@rmccue let me know if there are any updates on those plugins in the description.

[dashaluna]

Waiting to see what to do with both plugins as they are not on packagist.

[roborourke]

@dashaluna if you have time and want to come back to this you can create issues on the repos to request that they publish them on packagist. We've done that before.

[dashaluna]

Post cache: Automattic/advanced-post-cache#19
Lightweight term count: Automattic/lightweight-term-count-update#18

[roborourke]

Closing out in favour of #207 and #206