Poor theme performance due to calling Sensei `get_courses_average_grade()`.

[dd32]

The learn theme makes uncached calls to get_courses_average_grade() in the Course sidebar that causes a rather slow SQL query on certain pages.

This is mostly visible with courses that have been attempted by a lot of users.

Sensei code:
https://github.com/Automattic/sensei/blob/d1596603cd10e88260cd7f84831edcc9a137b203/includes/reports/overview/services/class-sensei-reports-overview-service-courses.php#L84-L108

We should ideally:

• Cache the value on the post type (Either as postmeta, or in object cache)
• Either calculate the average on a cron (postmeta), or update it upon course completion (postmeta, object cache + ondemand)
• Switch to a stored Sensei meta value if one exists

[dd32]

Upstream: Automattic/sensei#7765

[pkevan]

Created #3147 since the call was in the theme.

[dd32]

I'm going to leave this open, even though it's filed upstream, and the above PR has merged and fixed the most visible cases..

If you hit a cold cache on that value the request can take 30s on some courses, which is a horrible experience.

[pkevan]

If you hit a cold cache on that value the request can take 30s on some courses, which is a horrible experience.

Agreed - the PR was the easiest solution I could figure out given a lack of upstream movement.