We're in an age where websites happily serve up 8mb JavaScript bundles to display their static homepage to a mobile browser that has maybe a couple hundred MB allocated to caching things. Sure, it may be there if you immediately load another page on that site (and the devs aren't using a bundler that serves a different 8mb next time).

But if they leave and come back, they'll get the same full download experience.

Caching effects are much smaller on Safari with its per-domain cache, but even there if something broke caching for us it would be a high priority to fix.

(I work at Google on ad tags)

Thanks for that. It explains why even on very high-end new phones, switching between complicated web sites in different tabs causes a reload.

This is because your UAID is part of the URL to download the GTM snippit.

The old-school analytics.js file can be cached, though.

Someone correct me if this isn't true.

If you are that concerned then your page views should be sent from the server side.