boxtops, Barmaid is giving away trade secrets !LOL It's common for lawyers to experience 'overnight' instructions.
But just about the first thing lawyers learn in articles or as pupil barristers is how to read a file or brief. The seniors seem to have control of it in twenty minutes, however big it is, when juniors have just reached page forty, and probably haven't worked out what they're being asked. They learn what to look for, what it is being asked, and then where to find the key statements or letters, and then go on. They'll skim over the rest of it, just identifying the nature and relevance of most of the documents.
But , for all that, the lawyer does read quickly. And 'last minute' instructions are the bane of advocates' lives, but inevitable because the original advocate concerned may be unavoidably occupied in another court, or an application may itself be an emergency one.