Māṇḍūkī Śikṣā 5

5

doṣāprakāśastu vilambitāyāṃ varṇā drutāyāṃ na tu sūpalakṣāḥ
tasmāddrutāṃ caiva vilambitāṃ ca ttyaktvā naro madhyamayā prayuñjyāt

Faults are not manifested in the slow tempo, but in the fast tempo speech sounds are not easily discerned; / therefore, abandoning both the fast and the slow, a man should recite in the medium tempo.