They don't have troubles per-se, but think their system is fast enough to put the game on the fastest setting, and then expect to run in the blink of an eye.
It doesn't work quite that way
Each game hour the game engine has a myriad of calculations to perform. Each time the game clock advances, these calculations are fired off, and take, based on the system configuration, a certain amount of time to finish.
When you increase the game speed setting to produce a time-between-game-hours that is less than (or equal to) the time needed to finish these necessary calculations, you won't see a quicker advancement of the game clock anymore. Contrary to a chess program, these calculations are not aborted when the time is up. Instead, they are run through completion.
The game clock advances with a time interval that has the maximum of two times, the time needed to calculate all that the AI needs, and the wait time between game hours. Only the latter one is controlled by the game speed setting.
Now, what normally happens is that the game speed setting's time interval is bigger than the time needed to calculate the AI. Which means that after the AI has finished whatever it needed to do, there is time remaining before the clock advances. That is the time window where the game reacts to user interaction. Key strokes, mouse clocks, whatever. When you increase the game speed to the point where this remaining interval is reduced to zero, then the game will react extremely choppy. Because it is busy (almost 100% of the time) in running calculations for the AI.
