It is horrible for latency to use a US carrier roaming in China but without an immediate frame of reference people probably don’t notice. Those roaming with us sims are probably mostly accessing us websites so the latency isn’t going to be much different than accessing the site from local Wi-Fi assuming it’s not blocked. Where it really shows is trying to access Chinese sites in China roaming with a us sim.
If you want low latency roaming in China, state owned China Mobile will sell you a solution for that:
Just because you have a US IP address, doesn't mean traffic actually gets routed through it.
My limited understanding (from one uni course) is mobile devices get a secondary identifier that the networks use for the actual routing process. While a device may officially identify as having a US IP address, the cell carriers communicate a secondary, current IP address/identifier local to the device's current position.
It's much like how DNS works. You don't route all traffic through a DNS server. You send it a readable name, then communicated directly with the IP address that's returned.
This is how your mobile data traffic is served by your operator:
In 3G/LTE network there are terms called Gn interface, Gi Interface, Gp interface and many others.
Gn is to manage your mobility.
From Gi you will get local ip address and internet access.
From Gp you will be routed to you own cellular operator Gp interface.
When you are on your original cellular operator network, you will be served by Gn and Gi interface for your internet access.
When you are roaming, you will be served by Gn and Gp interface of local cellular operator that will forward to your original cellular operator’s Gp. After that it will routed to Gi interface of your original cellular operator.
It is pretty horrible for latency. Despite T-Mobile's free international roaming (albeit at reduced speeds), I'll usually get a local SIM card if I'm in the country more than a week because the latency can make it unusable sometimes.
IIRC the carriers do this for billing/metering reasons; otherwise they wouldn't have any idea how much data you're using while roaming, and would have to blindly trust whatever the partner carrier tells them.
This allows both the home and "visiting" operator to charge money. Also there's a (probably expensive) dedicated global network for these operator connections.
What I'd like to see is way for the visiting operator to charge little and route my packets directly to internet. I remember reading that this mode is also supported by the specs
Assuming you’re using it to access websites or services with PoPs in US/EU then tunneling traffic via US/EU VPN would add negligible latency, actually would probably improve your latency overall.
If you want low latency roaming in China, state owned China Mobile will sell you a solution for that: