I'd been using an easy-to-debug but glacially slow homebrew algorithm to generate the puzzles. With a little research, I discovered Eller's Maze Generation Algorithm (because a ConFusebox level is really a maze, only with lines where the path should be), which is a truly wonderous (although not well documented) thing. Basically it can generate mazes of arbitrary length with very low computing resources (at most three array-lookups per cell) and no more storage than is required to store a single row --not even the entire maze.
And that's important in Flash, as the Flash Player wants to shut down your app if you spend more than five seconds on a single frame. I had to distribute my maze-generation algorithm over four frames with a progress-indicator to let you (and Flash) know that things aren't crashed, and that just made things ugly.
With Eller's, however, things are so fast that putting it in a single frame isn't a problem. Generating a 12x12 maze my old slowass way took 3 seconds on the main development box. Now it's instantaneous.
What this means for you the player is that doing an "X-treme ConFusebox" with hugeass 15x15 mazes isn't going to tie your machine up for a half-minute generating the puzzle.