[MrFlippant]

I agree with Sean.
The best way to use pace cars is to make sure the pace cars are significantly "better" than the car you're driving. For example, in order to have pace cars be anything more than traffic, one should drive a magless car with stock tires, and the pace cars should have at least one standard magnet and grippy tires like silicone, urethane, or super-grips. This way, you can have the pace cars making laps at YOUR fastest (in the magless car), which is well under their capability. Thus, they won't be tailing out or coming off, and when SSDC speeds them up because of the track call delay time being added onto their lap time, they'll still be able to stay on until it slows them back down again.
If you have your pace cars close to their edge of performance, then that additional speed will cause them to tail out causing higher lap times, or come off, causing another track call, and the spiral heads down fast. Having a track call every lap will make those cars keep speeding up lap after lap, so try to avoid track calls as much as possible. If you're track calling almost every lap, well... something needs to change.

 

[MrFlippant]

The two seconds is the time provided to cars that might be coasting across the lap sensors as they are shut down. With magnetized minis, they'll stop on a dime, but with other cars, coasting can be quite pronounced. The two seconds is actually the length of the track call beep, but the sensors keep watching for cars during that time, just in case one crosses during the beep. Without this, cars might miss laps during a track call. The clock has to keep running during that beep, though.

What would be nice is is SSDC would ignore lap times after a track call, or maybe remove a second or two from lap times due to track calls. I don't know how SSDC 5.x and the APB manages this, though.