Trebuchets could fire rocks at incredibly high speeds, but there's no single answer because their power depended on a number of factors:

* Size: Larger trebuchets with longer arms and heavier counterweights could launch projectiles faster.

* Counterweight: The heavier the counterweight, the more potential energy it held, and the faster the projectile would be flung.

* Projectile size and weight: Larger, heavier rocks would naturally be slower than lighter ones.

* Arm length: Longer arms translated to a greater arc and higher velocity.

However, here are some estimates:

* Medieval trebuchets: Some historians estimate that medieval trebuchets could launch projectiles at speeds of 150 to 200 feet per second (46 to 61 meters per second).

* Modern recreations: Modern trebuchets built for competitions can achieve speeds of over 300 feet per second (91 meters per second).

For comparison, a baseball thrown at a professional level might travel at around 90 miles per hour (40 meters per second).

Important to note: These are estimations based on historical accounts and modern recreations. It's impossible to know with certainty how fast a specific trebuchet could fire.