I tested eSIMs across a multi-country trip: results

One eSIM, four countries

This is a real field report, not a lab test. We carried a single travel eSIM across four countries (Thailand, Japan, South Korea, and Vietnam) and ran 39 speed tests along the way in June 2026, all on 5G, at the kinds of places you actually use data: airports on arrival, city centres, temples, train and bus stations, beaches, and rural spots. The question we wanted to answer is the one every multi-country traveller has: does one eSIM really work well everywhere, or does it fall apart the moment you cross a border?

The short version: it held up across all four countries, connecting to a strong national network in each, and never once hit a dead zone in 39 tests. What changed from country to country was speed, not whether we had a signal. Overall median download was 82.5 Mbps, with a mean of 111.7 Mbps and a range from 17.9 to 478.0 Mbps. The country-by-country story below is where the useful detail is.

How each country performed

Thailand was the fastest leg and the most tested, 19 readings with a median of 112.0 Mbps. City 5G was excellent: Bangkok Chinatown topped out at 478.0 Mbps, and Bangkok Siam BTS Station hit 186.0 Mbps underground on the transit line. The islands were slower but always working, which the next section covers. Japan came next at a median of 79.9 Mbps over 12 tests, strong in the cities, Tokyo Haneda Airport reached 221.0 Mbps, with slower readings at a couple of regional airports.

South Korea was the steadiest country of the trip: 7 tests, a median of 69.4 Mbps, and a tight range from 58.7 to 82.5 Mbps, so we rarely saw the swings that Thailand and Japan showed. Even the DMZ Demilitarized Zone returned 63.7 Mbps. Vietnam was the shortest leg, a single test at Ho Chi Minh City Airport of 127.8 Mbps on arrival, which we report as one measurement rather than a country verdict.

What a multi-country traveller can take from this

The practical lesson is that a single travel eSIM covering a region can genuinely carry you across several countries without the setup churn of buying a local SIM in each. Across our four countries the same eSIM stayed connected on strong networks, AIS in Thailand, NTT Docomo in Japan, LG U plus in South Korea, and Vinaphone in Vietnam, and delivered usable-to-excellent 5G at every stop. For a trip like this, that convenience is the whole point, and our numbers suggest you do not pay for it in dead zones or unusable speeds.

That said, speed does vary by country and by spot, so set expectations accordingly. Expect fast city 5G in Thailand and Japan, steady mid-range speeds in South Korea, and slower but still working connections at airports and remote areas anywhere. Our slowest reading across the whole trip was 17.9 Mbps at an island pier, still enough for maps and messaging. If your itinerary is heavy on cities, you will rarely notice a limit; if it leans rural, plan for lower speeds without expecting outages.

How we ran these tests

For transparency, here is the method. Every reading was taken on our own travel eSIM as the active data line, on 5G, using a standard speed test at the location named. We logged download, upload, and ping where available, along with the place, the host network, and the date. We did not cherry-pick fast spots; the 39 tests include airports, undergrounds, rural piers, and rice fields precisely because those are where coverage claims are usually tested hardest.

We report medians rather than averages as the headline, because a few very fast city readings, like the 478.0 Mbps in Bangkok, pull the mean up and away from typical experience. And we show the sample size on every figure so you can weigh it. These same measurements feed our Mobile Speed Index, which turns field data like this into a comparable score by country and operator, linked below if you want to see how a given destination scores before you travel.

Frequently asked questions

Does one eSIM work across multiple countries?

In our test, yes. A single travel eSIM covered four countries (Thailand, Japan, South Korea, and Vietnam) with a working 5G signal at every one of 39 locations, re-attaching to a local network at each border with no swapping or re-buying. Speed varied by country but coverage did not fail.

How did speed change between countries?

Median download ran from 69.4 Mbps in South Korea to 112.0 Mbps in Thailand. Japan sat at 79.9 Mbps. Cities were fastest, airports and remote spots slowest, but every location stayed usable across the trip.

Which country was fastest on the trip?

Thailand, with a median of 112.0 Mbps over 19 tests and our fastest single reading of 478.0 Mbps in Bangkok. South Korea was the most consistent at 69.4 Mbps, with the tightest range of the four.

Did the eSIM ever lose signal between countries?

No. Across all 39 tests in four countries, we never hit a dead zone. The slowest reading was 17.9 Mbps at a Thai island pier, low but still usable. Border crossings changed the underlying network without interrupting our data.

Is this a big enough test to trust?

It is a real field sample with sizes shown. Thailand (n=19) and Japan (n=12) are well covered; Vietnam is one reading and labelled as such. We show every number's sample size so you can judge how much weight to give it.