Game Speed is the speed at which your game runs. There are 5 speeds to chose from: Slower, Slow, Normal, Fast and Faster. Faster is the default speed for custom and ladder matches. In the campaign, the game speed cannot be altered when playing at Brutal difficulty.
All tooltips are written based on Normal speed so when playing on Faster, they are incorrect in terms of real-time seconds. For example, the MULE tooltip says that it lasts for 90 seconds, but in a standard game on Faster, it will last 65. The way the game handles speed changes is simple. Instead of having different build-times coded between speeds, the game will run faster or slower depending on the speed chosen.
The inaccuracy with timing and tooltips can also be seen during replays. When you finish a game, it will tell you how long the match lasted in real-time. However, when you load the replay, it will display the length of the game as if it was played on Normal speed. When calculating APM in replays, the game will take the actions per in-game minute. This means if you are playing on Faster, it will be calculating your actions per 43 seconds or so.
A rough guide for the differences between speed is below. This was calculated using the 90 second MULE shut-down time.
|Gamespeed||# seconds to shut down||% of standard time||x normal speed|
Tamerlane gives detailed measurements
However, due to the nature of deterministic synchronous game engines, the actual real-time values cannot be calculated exactly. The game attempts to keep constant 0.6x, 0.8x, 1.0x, 1.2x and 1.4x time modifiers according to the crash logs, but due to network and processing delays the actual observed time can easily shift 1-3% over a given time period.