Is PageSpeed Insights bypassing Google CDN cache?

Question

We're using Google Cloud Platform to host a WordPress site:

Google Load Balancer with CDN -> Instance Group with single VM -> Nginx + WordPress

From step 1 (only VM with WordPress, no cache) to the last step (whole setup with Load Balancer and CDN) I could progressively see the improvement when testing locally from my browser and from GTmetrix. But PageSpeed Insights always showed little improvement.

Now we're proud of an impressive 98/97 score in GTmetrix (woah!), but PSI still shows we're pretty average, specially on mobile (range from 45-55).

Problem: we're concerned about page ranking in Google so we'd like to make PSI happy as well. Also... our client won't understand that we did make an improvement while PSI still shows that score.

I was digging and found a few weird things about PSI:

• When we adjusted cache-control in nginx, it was correctly detected by local browser and GTmetrix, but section Serve static assets with an efficient cache policy in PSI showed the old values for a few days.
• The homepage has a background video hosted in 3 formats (mp4, webm, ogv). Clients are supposed to request only one of them (my browser and GTmetrix do), but PSI actually requests the 3 of them. I can see them in Avoid enormous network payloads section.
• When a client requests our homepage, only the GET / request reaches our backend server (which is the expected behaviour) and the rest of the static assets are served from the CDN. But when testing from PSI, all requests reach our backend server. I can see them in nginx access log.

So... those 3 points are making us get a worse score in PSI (point 1 suddenly fixed itself yesterday after days since we changed cache-control), but for what I understand none of them should be happening. Is there something else I am missing?

Thanks in advance to those who can shed some light on this.

Answer 1

but PSI still shows we're pretty average, specially on mobile (range from 45-55).

PSI defaults to show you a mobile score on a simulated throttled connection. If you look at the desktop tab this is comparable to GT Metrix (which uses the same engine 'Lighthouse' under the hood without throttling so will give similar results on Desktop).

Sorry to tell you but the site is only average on mobile speed, test it by going to Performance tab in developer tools and enabling 'Network:Fast 3G' and 'CPU: 4x Slowdown' in the throttling options.

Plus the site seems really JavaScript computation heavy for some reason, PSI simulates a slower CPU so this is another factor. One script is taking nearly 1 second to evaluate.

Serve static assets with an efficient cache policy in PSI showed the old values for a few days.

This is far more likely to be a config issue than a PSI issue. PSI always runs from an empty cache. Perhaps the roll out across all CDNs is slow for some reason and PSI was requesting from a different CDN to you?

Videos - but PSI actually requests the 3 of them. I can see them in Avoid enormous network payloads section.

Do not confuse what you see here with what Google has used to actually run your test. This is calculated separately from all assets that it can download not based on the run data that is calculated by loading the page in a headless browser.

Also these assets are the same for desktop and mobile so it could be for some reason it is using one asset for the mobile test and one for the desktop test.

Either way it does indeed look like a bug but it will not affect your score as that is calculated in other ways.

all requests reach our backend server

Then this points to a similar problem as with point 1 - are you sure your CDN has fully deployed? Either that or you have some rule set up for a certain user agent / robots rule set up that bypasses your CDN. Most likely a robots rule needs updating.

What can you do?

1. double check your config, deployment etc. Ensure it has propagated to all CDN sites and that all of the DNS routing is working as expected.

2. Check that you don't have rules set for robots, I notice the site is 'noindex' so perhaps you do have something set up while you are testing things that is interfering.

3. Run an 'Audit' from Developer Tools in Google Chrome -> this uses exactly the same engine that PSI uses. This may give you better results as it uses your actual browser rather than a headless browser. Although for me this stops the videos loading at all so something strange is happening with that.