FUNDAMENTALS OF GAME DESIGN, SECOND EDITION
Races and Time Pressure
In a race, the player attempts to accomplish something before someone else does, whether that involves a physical race through space or a race to create a structure, to accumulate something, or to do practically anything as long as the game can distinguish which player finishes first. Normally we think of races as peaceful, involving competition without conflict, but races can be combined with fighting and many other kinds of challenges.
Time pressure discourages careful strategic thought and instead encourages direct, brute-force solutions. With only 15 seconds to get through a host of enemies and disarm a bomb, the player won't stop to pick off enemy units one by one with sniping shots; he's going to mow them down and charge through the gap, even if that means taking a lot of damage.
Time pressure increases the stress on a player and changes the feeling of the game - play considerably, sometimes for better and sometimes for worse. In something like a car racing game, time pressure is an essential part of the experience, but use caution in adding time pressure to challenges that aren't ordinarily based on time constraints. You will deter some players entirely and make the challenge more difficult in any case. To keep the absolute difficulty level constant, whenever you increase the time pressure on a player, you should also reduce the amount of intrinsic skill required.