This is only true if these latency outliers are evenly distributed over time, if they indeed happen once every 30 minutes than only users requesting a page at the same time will be affected.

Excellent point, yes.

That ignores that the 99th percentile is all for the dynamically generated content, rather than any of the scripts/stylesheets/images which should be distributed close to the user on a CDN

Sometimes even to create the HTML you serve itself, many DB calls are performed: tens is not very uncommon.

But the root of this thread was talking about users encountering 99th percentile for at least one resource on the page. But the common pattern is that most of the resources on a page are static (even if the generation of the page itself is dynamic).

I'll note that I'm excluding API calls here. I don't commonly hear API calls referred to as 'resources' on a page, so I'm assuming that this wasn't being referred to.

XHRs appear under the "Resources" tab of my web inspector, so I guess by that right they're resources on the page :) But you're right -- the latency histograms for static resources are very different than for dynamic resources.

So while the probabilities do suggest that most users will encounter at least one 99th percentile latency, it's very different if that latency is for a static resource vs a dynamic one. Thanks for pointing it out.