What's a stall speed?

A friend of mine uses a 4500 stall speed convertor, but this is in a car that produces around 900 horse and runs the 1/4 in 6.9 sec.
The engine won't idle under 1800 rpm, so the stall speed has to be higher. Also using a higher stall speed allows the engine to reach higher rpm and produce more torque when it does transmit power. The car will lauch much quicker when launched at the stronger power curve. This is annoying on the street. It is very difficult to cruse with a car with a high stall speed, and as stated, low end response is awful. Mate the convertor to the engine, and what you plan on doing with the car.