T-Mobile USA has begun throttling mobile hotspot data when its network is congested while giving priority to smartphones and other devices that connect directly to the cellular network.

T-Mobile has been notifying customers of the change yesterday and today with a message that says, "We just made your network better again" and that "T-Mobile device data comes first."

"We've primed the network for on-device use," the carrier says on its website. "So now when there's congestion, you may notice higher speeds for data on your T-Mobile devices versus Smartphone Mobile Hotspot (tethering)." Prioritization of on-device data is triggered "at times and at locations where there are competing customer demands for network resources, which may result in slower tethering speeds," T-Mobile also says.

That means your smartphone should still be fast, but devices like laptops that connect to the phone's mobile hotspot will get slower Internet access.

T-Mobile is making this change as it tries to shift customers from data buckets to plans that are nominally "unlimited" but in reality have several limits. The recently unveiled T-Mobile One plan has no monthly data cap or overage fees, but it throttles video to 1.5Mbps (enough for about 480p resolution) and throttles other data usage when customers who have used more than 26GB in a month connect to congested cell towers.

The standard T-Mobile One plan for $70 a month also limits hotspot speeds to 512kbps at all times, much slower than T-Mobile's un-throttled 4G LTE tethering download speeds that are typically 3 to 25Mbps with peaks of up to 90Mbps. T-Mobile One customers can pay an extra $25 ($95 per month total) to unlock high-definition video and unlimited hotspot usage at 4G speeds. But even if you're paying for the "unlimited" high-speed tethering, the hotspot data will be slowed down when the network is congested.

T-Mobile confirmed to Ars that the new policy applies to all plans, including T-Mobile One and Simple Choice.

We asked T-Mobile what tethering speeds customers can expect while phone data is prioritized; the company did not provide a specific answer, but said tethered devices would see "relatively" slower speeds compared to smartphones. That could mean tethering speeds would still be above 512kbps unless network congestion is particularly severe.

A T-Mobile FAQ said that customers "probably" won't notice any changes.

"In the vast majority of times and places, you should notice little if any difference between your T-Mobile device and Smartphone Mobile Hotspot data speeds," T-Mobile said. "But in the small number of times or locations where there's network congestion, you may notice reduced data speeds with using Smartphone Mobile Hotspot."

The new policy was put in place in early September and described on T-Mobile's Open Internet Disclosures page, though the notice to customers just started going out this week. That webpage notes that "mobile hotspot features are intended for personal mobile connectivity [and are] not intended to be a complete broadband replacement for multiple users over an extended period of time."

The new policy will also apply to older plans that have monthly data limits and an older plan that provided unlimited data on phones plus 14GB for tethering each month. (If you're a T-Mobile customer without an "unlimited data" plan, all of your Internet usage is throttled to 128kbps for the remainder of the month after you hit your data limit unless you purchase additional high-speed data.)