Since ANY roaming device data will terminate in the originating country you should NOT run speed tests with default settings. If you do so you are attempting to run a speed test NOWHERE near where your device gets its Internet connection.

When you roam ALL your data is routed via TMO in the US (this is why you can also avoid Internet blocking in countries like China). Since all data is routed back to the US via the partner roaming network to TMO's data network ALWAYS turn off GPS/location services and let the speed testing app use IP geolocation (or manually select a server in the US if you can). This is the only way to get a true latency test. Without doing this you are adding latency from T-Mobile US data network BACK to where you are across the internet which can add roundtrip 300-500ms - that extra 300-500 ms is NOT actually what you are getting as you go:

[Your device] -> [Local carrier] -> [Local carrier routes to TMO] -> [TMO data service to Internet].

Assumptions to make

[Your device to local carrier]: 30-140ms depending on if UMTS to HSPA+

[Local Carrier to TMO]: Expect 20-200ms depending on where you are and if that local carrier has good interconnect back to TMO. (e.g. I was in Korea and switched from SKT to KT/Olleh and this part of it went down 150ms) - so local carrier choice can make a BIG difference

[TMO data to Internet]: Assume 5-10ms

When you test to a local server in the country you are in you force another 150-250ms EACH way (300-500ms roundtrip) to a server that is actually nowhere near to where your internet terminates.