Normal cellular function relies on the efficient and accurate readout of the genetic code. Single-molecule experiments show that transcription and replication are highly intermittent processes that are frequently interrupted by polymerases pausing and reversing directions. Although intermittent dynamics in replication are known to result from proofreading, their origin and significance during transcription remain controversial. Here, we theoretically investigate transcriptional fidelity and show that the kinetic scheme provided by the RNA-polymerase backtracking andtranscript-cleavage pathway can account for measured error rates. Importantly, we find that intermittent dynamics provide an enormous increase in the rate of producing long transcripts of high fidelity. Our results imply that intermittent dynamics during transcription may have evolved as a way to mitigate the competing demands of speed and fidelity in the transcription of extended sequences