~2-3 ly/day, as is standard for a wartime slipspace drive.
Discounting the slipspace engine?
99.9(repeating)% the speed of light.
In space, the only barrier to how fast one can go is mass-energy equivalence.
The Pillar of Autumn can never reach a "top speed." Provided it had the fuel, it could literally keep accelerating, forever.
A ships speed is not classified by its maximum speed because their is no such thing as maximum speed in space, a ships speed is classified by how long it takes to reach high speeds.
We can calculate the speed using the intro cutscene of Halo 2 (Classic) which shows the (Marathon CG Model) Pillar of Autumn going as fast as it can to get through the Covenant Blockade. It goes from being a small blip to being a full Ship model in 1 second. Using Reach as a basis we can determine the speed. Unfortunately Idk how to do that.