Latency is additive, so all that copper coax that and mux/demux in between a sizeable chunk of Americans and the rest of the internet means you're looking at a minimum roundtrip latency of 30ms if server is in the same city. Most users are also on Wi-Fi which adds and additional mux/demux + rebroadcast step that adds even more. And most people do not have the latest CPU. Not to mention mobile users over LTE.
Sorry, but this is 100% a case of privileged developers thinking their compute infrastruction situation generalizes: it doesn't and it is a mistake to take shortcuts that assume as such.
uh have you ever tried pinging a server in your same city? It's usually substantially <30ms. I'm currently staying at a really shitty hotel that has 5mbps wifi, not to mention I'm surrounded by other rooms, and I can still ping 8.8.8.8 in 20ms. From my home internet, which is /not/ fiber, it's 10ms.
If you've ever used speedtest.net, you've almost certainly been using benchmarking against a server in your own city, or at least as close as your ISPs routing will allow. Ookla servers are often specifically optimized for and sometimes hosted/owned by ISPs to give the best possible speeds. Google's DNS servers use anycast magic to get similar results. Basically no service you actually use outside of very, very large providers is likely to be anywhere near you an won't get that kind of latency, even with a very good ISP and LAN.
10ms is a best case for DOCSIS3.0/3.1, it means you have near optimal routing and infrastructure between you and the node or are using some other transport like ethernet that is then fed by fiber. I currently get 24ms to my local Ookla host a couple of miles away over a wired connection with a recent DOCSIS3.1 modem. Hotel internet is likely to be backed by business fiber. They're likely throttling you.
I worked for an ISP for several years, there's a huge range of service quality even within the same provider, zipcode, and even same location depending on time of day.
if I'm able to ping 8.8.8.8 in 10ms, doesn't that mean that my rtt latency to get out of the docsis-part of the network is 10ms, and that any latency beyond that will be the same regardless of docsis/fiber?
Yes, the majority of that latency is likely coming from DOCSIS. You're likely in a major city, large enough that Google has an anycast DNS server nearby.
latency != throughput