SpeedSims are simply scripted; they are all scripted to be faster than a DarkSim. All SpeedSims have the same speed: quite fast. Their speed can be increased further through the "Fast Movement" option. The accuracy and behavior are influenced by their difficulty.
SpeedSims are extremely proficient in objective-based modes:
In Hold the Briefcase, SpeedSims can effectively outrun EVERY opposition.
In King of the Hill, SpeedSims can easily race to and capture the hill, they strafe so quickly that its hard to hit them. Against any CPU, however, accuracy is not a matter.
In Hacker Central, the SpeedSim can quickly get the Uplink, and race to the computer.
Probably the most deadly mode for it is Capture the Case. It can rack up points very easily.
If you are attacked by a SpeedSim, try to move close to it and circle it, if it isn't moving forward or backwards, it's much easier to hit.
A Player C-Strafing can actually outrun a SpeedSim, and consequently, DarkSims!