I found out about this feature by watching the TAS for the game: http://tasvideos.org/forum/viewtopic.php?t=22696
After entering the Level Select screen (press select), you can hold Select, and press A/B to change an undisplayed Game Speed variable. A increases speed, B decreases speed. Pressing Select+A 34 times will select the highest possible speed, where the game becomes unplayably fast. (More than 34 times wraps back to very slow speeds)
And it also turns out that the game's Instruction Manual even documents this feature: 
Great info, thanks! I think speed increases should be allowed in this speedrun, the game will be much harder and faster. And each person will be able to choose their own level of speed, as in Tetris.
Very interesting. My initial reaction is that this should be allowed, as it is a natural game function available to all players, but I think a new category should be made, and the current category should allow no speed changes. As Neural89 said, people will choose their own speeds, which will cause the any% record to plummet extremely low. It definitely seems to me like the kind of dynamic that warrants a separate category. Nice find Dwedit!
I think the rules for the new category must also indicate that not only a one-time increase in speed is allowed, but a decrease in speed and the ability to completely vary the speed before each level throughout the entire run. It shouldn't just be a category of running through the entire game at some constant increased speed compared to the default speed. Rather, it should be a “category with freely variable speed” - it seems to me that this gives much more different strategic solutions than a constant speed. In addition, there may be people who want to do the run just at a slightly reduced speed compared to the default speed. This category will provide an opportunity for them to make their own runs too.
