Time lag or delay between the source text input and the interpreter’s target text is known as Ear-Voice-Span (EVS) or décalage. Time lag has also been measured in written translation based on eye-tracking and key-logging hence called Eye-Key-Span, (EKS). Time lag provides insight into the temporal characteristics of simultaneity in interpreting, speed of translation and also into the cognitive (perceiving) load and cognitive processing involved in the translation/interpreting process.
Simultaneous interpreting is a complex task involving several cognitive processes: speech comprehension and production, memory, attention/resource allocation and coordination.
The first effort is the listening and analysis effort which includes the detection and identification of stimuli and the assignment of a meaning to what is heard.
Simultaneous or consecutive interpreters tend to work close to cognitive saturation, which means that omissions or accuracy can be caused because attentional resources required to perform adequately were not available for a particular comprehension, memory storage or retrieval or production task at a time when they were needed. Some estimate that the optimal moment for the interpreter to start speaking is immediately after all syntactic (refers to grammar) and semantic (refers to meaning) ambiguities in the unit have been resolved. Others found that interpreters usually wait for the predicate (the part of a sentence or clause containing a verb or stating something about the subject) of a sentence before they start interpreting it.
Indeed, a long EVS might mean that the interpreter prioritizes the listening effort over the production effort, or the rationale being that longer EVS probably reflects processing difficulties. While a short EVS potentially means that the interpreter is saving working memory capacity. However one factor that works significantly better, is advance preparation, this being reflected both in accuracy and in ability to maintain a shorter EVS/EKS.