>How long did it take x user to navigate from x screen to y screen is one of the most valuable metrics for any site

Stats like that are only used to implement dark patterns better and justify user hostile decisions since pretty much the time the idea of telemetry was introduced. Otherwise, we'd live in the world of perfect web ui and we're not.

That can be implemented. Within a session you don't need to know it's the same person tomorrow, so a per-day key derived server-side is enough to measure that someone took 40 seconds from x to y. No cookie, no localStorage, nothing stored on the device, nothing to consent to. Hash ip + user agent + your domain with a secret salt that rotates and is destroyed every 24 hours, and you are on the safe side. Of course, recognizing users across days requires consent. But is that really necessary?

In the world of zscaler, CGNAT & corporate proxies, IP address is nowhere near enough.

Or use some JS to put the time-on-page in the next request, right?

Is it a violation to send data that could theoretically be used for more invasive tracking than you actually do? I don't think so, or else you'd need consent just to receive an IP packet.