It is said that the Buddha spoke very quickly. How
quickly? During the time an ordinary person speaks one word
the Buddha could speak 128 words. Buddha spoke very fast.
Source: Handbook of Abhidhamma Studies, by Venerable Sayādaw U Sīlānanda, Volume 1, page 19.
So it might be OK to listen fast but if you can comprehend what is been said. Perhaps to 128x times faster. But remember though the Buddha spoke (I would say communicated) 128 words when someone else spoke 1 word, each listener would hear a subset as if this has been said to him. So in this case the amount you can speedup is something less than 128x but also limited by the rate you can understand what is spoken and also comprehend what is spoken. If you can
- understand what is spoken and
- comprehend what is spoken
at the rate of 2x speed then it is OK.