And this is the key problem I've seen over and over. When you get a pre-gen, to run it well you need to:
1) Read it through.
2) Understand it - which often requires taking quite a lot of notes, because a lot of pre-gens are really badly written, and really badly organised. Vital description or information that gives a scene its entire context can be a dozen pages away (often later!) in a text box, or worse, in the middle of a paragraph about something else!
3) Make any changes to make to make it "not suck". Hopefully there is no 3. A lot of smaller adventures, there is no 3. They're just solid little adventures, and you do little or nothing to make them work (maybe change a town name or something, or otherwise recontextualize them). With big adventures? There's almost never not a 3, and sometimes 1+2+3 here is significantly more than writing your adventure, often for a lesser outcome.
Does that make sense?