PSI isn't requesting all referenced images

Hi Maybe is a dumb question, but… Regarding to native lazyload images in chrome, the viewport is spanded depending on the connection speed. So whats happens when the throttling is “simulated”? Thats is that I can see: In a chrome version 80 with Nexus 5x emulation: Whit native lazy load disbled, only the suplied (lazysizes) lazyload works, so in my site, only 3 images are loaded until you scroll down. When do the audit, only these 3 images are loaded. When native lazyload is enabled, 7 images are loaded, no matter if in performance panel I set network and cpu throttled or not.

In PSI or web.dev the behaviour changes, same on WPT: Some times it load 3 images, some times the 7 ones.

These behaviour changes the timming for tests? At least the onLoad event is at a diferent time…

EDIT: I´m using a WP plugin that uses lazysizes but put the loading=lazy label and a js snippet that bypass the lazysizes if native lazyload is in place.

Images, the same url, 2 times PSI run:

[connorjclark]

Simulated throttling means we don’t throttle as the page is loading, we only throttle in our analysis step. However, I could see that our backend services have a varying quality of connection type. That may cause the behavior you see herre. Lazy loading radius is based on ECT.

@exterkamp

We use a brand new chrome profile for each run. We don’t understand exactly when ECT is determined. Ideally we should emulate this behavior. To investigate, we should log out ECT in both environments and see what the difference is.

[Lofesa]

In the same run PSI 6.3.0 show 5 images loaded in mobil emulation and only 2 on desktop

In others runs it show 2 images for both devices, mobil and desktop. In chrome devtools LH desktop, same… some times show 2, some times 5 and some times 6… In mobil seems that consistently only show 2 images Dev Tools LH 6.4.0

Dev Tools LH 6.0.0

I will let the site w/ native lazyload from lazysizes plugin enabled for 48H so you can test it. With native lazyload disabled the site loads only 2 images in PSI and all Dev Tools LH versions and Chrome.