Cache `count_user_posts()` #12542

Job	Run time
Core running from build / Core running from build / Linux	2m 4s
Core running from src / Core running from src / Linux	1m 56s
Core running from build / Core running from build / Windows	4m 43s
Core running from src / Core running from src / Windows	3m 46s
Core running from build / Core running from build / MacOS	1m 59s
Core running from src / Core running from src / MacOS	1m 49s
Gutenberg running from build / Gutenberg running from build / Linux	8m 36s
Gutenberg running from build / Gutenberg running from build / Windows	12m 14s
Gutenberg running from src / Gutenberg running from src / Windows	12m 51s
Gutenberg running from src / Gutenberg running from src / Linux	8m 35s
Gutenberg running from build / Gutenberg running from build / MacOS	13m 22s
Gutenberg running from src / Gutenberg running from src / MacOS	10m 57s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 22m 52s

Provide feedback