Sounds like you have a pulley ratio that turns the alternator slow.
The "coming on speed" of the alternator is a relatively fixed speed at the alternator. If you have the pulleys to where the alternator is turning slower (relative to the crank) than oe setups it will take more engine (crank) RPM to bring the alternator on line and extinguish that light.
If it's causing a problem you can use a smaller diameter alternator pulley, or a larger diameter crank pulley.
From the sound of it, you don't need one that's much different than you currently have.
