Back in the day (2001) I worked on the PalmOS version of Scrabble. One thing that surprised me when I was tuning the AI was that it played better when all words of seven or more letters were removed from its vocabulary. Apparently searching through the longer words was a worse use of time than searching for other placements of shorter words (except on the highest difficulty, where it was given a larger time budget).

We ended up giving the lower difficulty levels a restricted dictionary anyway, but this was to affect players' perception of the difficulty rather than the actual difficulty (the AI on "beginner" mode shouldn't be playing a lot of words you've never heard of). We adjusted time budgets so that difficulty still ramped up as the dictionary expanded at higher levels.