henry copeland:

Takes a long time to form a sentence

The brevity of these silences is doubly astonishing when you consider that it takes at least 600 milliseconds for us to retrieve a single word from memory and get ready to actually say it. For a short clause, that processing time rises to 1500 milliseconds. This means that we have to start planning our responses in the middle of a partner’s turn, using everything from grammatical cues to changes in pitch. We continuously predict what the rest of a sentence will contain, while similarly building our hypothetical rejoinder, all using largely overlapping neural circuits.
- www.theatlantic.com