During the English Google SEO office-hours from October 22, 2021, John Mueller was addressing a question (as he usually does during these sessions).
At around the 26:29 mark, John began reading and addressing a question about improvements and hopeful search results.
(Basically, a person who submitted a question said they had made improvements, and wanted to know how reliable the observed ranking improvements were. Were they a direct result of the changes made?)
You can watch that at the video below, which is queued to the appropriate time.
While addressing the question, John mentioned something important:
“One thing maybe worth mentioning, specifically about speed, is that for the Core Web Vitals, we take into account the data that is, I think, delayed by 28 days, or something like that.
“So about a month back. Which means if you make significant speed changes on your website that affect the Core Web Vitals, and accordingly the Page Experience ranking factor, then I would expect it to take about a month to be visible in the search results.”
I think it’s rare that you hear Google give a quantified number of days for anything pertaining to search engine improvements.
This is one of those rare times.
Take advantage of it