Simultaneous interpreting is a complex process that entails an instantaneous delivery of a message in another language using concurrent multi-tasking. A multitude of factors can influence this difficult process. Speed is one of the variables most frequently put forward as a major challenge that could overload interpreters' cognitive capacity and lead them to commit errors. Studies on the impact of high delivery rates on the performance of professional interpreters are scarce. Against this background, a study was conducted to shed light on the influence of speed on their rendition. Ten staff interpreters from the United Nations were asked to simultaneously interpret three speeches at different speeds. The findings indicate that at higher delivery rates interpreters arbitrarily omit more information, and that these omissions are longer, implying a loss of meaningful information. The results also show that interpreters omit short segments at a moderate delivery rate, particularly redundancies and synonyms. High delivery rates have an impact on the quality of interpretation, because if a speech is delivered at such a high speed that interpreters cannot adequately process the information, their interpreting output may be negatively affected. The chapter examines the implications that these findings have for interpreter training and the profession.