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SMS is limited to 1 message per 3-6 seconds. The modem incidentally limits throughput.

Also, Google Fi has unlimited text/call for $20/m. Their SIM does work with USB GSM modems.

GSM is not available on AT&T or Verizon and T-Mobile is ending GSM support in under a year. Right now T-Mobile's GSM is running in a 200Khz carrier in the guard bands of LTE, making it perform worse due to the narrow bandwidth which means your allocated fewer timeslots which limits message throughput, and interference from the LTE carrier directly beside it degrades signal quality as well.

Nevermind most GSM stacks crash when receiving/sending many messages quickly, they aren't designed for high throughput messaging.

Sending via a carrier's multi-device calling/texting option where you bypass their cell network and connect to their servers over the web, or using VoLTE or VoNR is going to perform much better than GSM

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