LookAhead isn’t speeding up appearance of translation results with large TMs connected

OK, so here's a question. Isn’t LookAhead supposed to look ahead? And yet, with it switched ON, if I connect a large TM to my project (with 2 million TUs), and click Ctrl+Enter, I still have to wait over a minute for the results to appear in the Translation Results pane. What's going on here?

Michael