The issue addressed in this study is the impact of source text presence on simultaneous interpreting performance in fast speeches. Fast speech rate is assumed to frustrate even professional interpreters in simultaneous interpreting (SI) without source text (ST) scenario, yet little is known about what happens when the ST is available to interpreters, an interpreting practice of increasing popularity. Previous studies present mixed results concerning the effect of fast speech rate and ST presence on SI quality, which further adds to the complexity of this issue. This study adopted the experimental approach with a qualitative assessment of SI quality, quantisation of output parameters and retrospective interview, to unravel some myths surrounding this issue. The 54 trainees were randomly divided into two groups to interpret two fast Chinese speeches into English under with and without ST conditions. The results found a significant effect of ST presence on the qualitative assessment of interpreting performance, on the interpreting delivery fluency, and on the information completeness. The facilitative effect of ST presence in fast speeches was confirmed while new issues also arise concerning trainee interpreters’ ability in eye-ear coordination.