You may observe a certain time the PageSpeed score is low, and a few minutes later it is again shining. This problem can occur mostly on the home page of the website, but the underlying cause can be on other pages of the website too. There are a few reasons why this may happen -.
Before checking the performance score, please ensure that the optimized version of the page exists and is being served to visitors. This can be easily verified in any browser as explained in the low score troubleshooting guide.
When a content change is detected on any page, the cached data for the modified page is cleaned. Since, a particular post or page can be referenced on the home page in many dynamic ways, such as title, summary, thumbnail, or pagination, the home page is purged too. This is done to prevent showing stale content on the home page. The downside is, that when any page is modified, the homepage is also purged.
After a purge, the optimized copy of the page is built again and there could be a very small time gap. If during this gap, performance is evaluated, it results in a low score as, during this time, the original version of the HTML page is served while the page is being optimized in the background. This strategy can be changed from the plugin’s Settings tab.
Under the plugin settings tab, the site owner can choose to either –
The same holds true in case any plugin gets auto-updated or manually which can change the page’s layout or has any CSS changes.
Many websites are having cache buster patterns in CSS which reduces the optimization effects. This can lead to fluctuations in PageSpeed since once a page is optimized with the current set of stylesheets, and it changes a moment later, it invalidates critical-css generated for the page, and also cached stylesheets.