A user's mobile device has slow data speeds. The user first noticed the stow speeds after downloading a large application to the device. The application download MOST likely caused:

A hardware issue would produce persistent and broad connectivity problems rather than symptoms that appear immediately after a specific download event.

Downloading an application does not degrade the device's cellular radio or network stack in a way that would reduce data throughput.

Most mobile carriers enforce a monthly data allowance; once a user exceeds that limit, the carrier throttles cellular data speeds significantly or blocks further data use. A large application download consumes a substantial amount of data in a short time, and if it pushed the user over their cap, the resulting carrier-imposed throttle would directly explain the slow data speeds observed afterward.

Downloading a large application does not modify the device's Wi-Fi settings or cause it to switch between Wi-Fi and cellular network configurations.