You can easily see this when using WiFi aboard a flight, where latency is around 600 msec at minimum (most airlines use geostationary satellites, NGSO for airline use isn't quite there yet). There is so much stuff that happens serially in back-and-forth client-server communication in modern web apps. The developer sitting in SF with a sub-10 ms latency to their development instance on AWS doesn't notice this, but it's sure as as heck noticeable when the round trip is 60x that. Obviously, some exchanges have to be serial, but there is a lot of room for optimization and batching that just gets left on the floor.
It's really useful to use some sort of network emulation tool like tc-netem as part of basic usability testing. Establish a few baseline cases (slow link, high packet loss, high latency, etc) and see how usable your service is. Fixing it so it's better in these cases will make it better for everyone else too.