Originally Posted by NickP 1K

You get to use whatever network technology the roaming network has enabled for roaming with T-Mobile, that being said it will normally be HSPA/3.5G (TMO brands it 4G without the LTE), or LTE, some rare cases you will fallback to EDGE (e.g. China if not using a TD-LTE or TD-SCDMA device).

So regardless of what network technology you get, the connection back to T-Mobile where all data is tunneled back to WILL max bandwidth throttle to 128Kb/sec (people on the new T-Mobile One + plan to 258Kb/sec). Better off picking an LTE network as latency (overhead on the network) will be lower, but all traffic is tunneled back to the US and some roaming networks have less capable ability to do this - hence why performance varies. Paid international data roaming packs will remove this throttling for the given amount of data on te pack.

Canada and Mexico on T-Mobile One or Simple Choice North America plans will be full speed using your domestic data plan allocation (no limit if on Unlimited - though throttling may occur for excessive use)