The slowdown on Thursday for people in the capital, Taipei, and six other cities and localities was part of the last phase of the island's annual Han Kuang exercises, military drills that began last week and aim to model a response to a possible Chinese military invasion.
Telecommunication operators slowed mobile web traffic to 256 kilobits per second, making it difficult to exchange anything other than texts.
Usually, residents can access 5G in major cities.
Taiwan's President Lai Ching-te apologised to the public for the inconvenience, "We ask for your understanding and support, as this is a necessary drill," he said.
Taiwan has stepped up the intensity of the Han Kuang drills, incorporating reservists and more civilians into the exercises with a focus on real-world scenarios.
Previously, urban resilience drills in Taipei went largely unnoticed as they were not disruptive.
For example, air raid drills in the metro station did not stem the flow of passengers.
In 2026, integrating civilians and co-ordinating with local governments was a key part of the exercises, according to the Taipei Times.
At a Taipei metro station on Thursday afternoon, passengers waited to be released after the station's gates were shut for the simulation as part of air raid drills.
Lai also visited a hospital as it simulated how to relocate its critical operations to underground facilities as part of the drills.
Taiwan is ramping up its defence exercises and purchases in response to heightened tensions with China, which claims the island as its own.
China sends military planes, as well as Coast Guard ships and other vessels towards Taiwan on a daily basis and carries out large-scale military exercises from time to time.
Between Wednesday to Thursday morning, 18 Chinese warplanes and 11 navy ships operated around Taiwan, the island's defence ministry said.
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