Hi RDD - no mock up attached. But I imagine its a 24" long slot in 3/4" timber. The fastest way would be to use an upcut tool thats smaller then the slot size and do it in one pass down the middle. Then do a finishing cut. Why use a downcut? they can only do a light cut so they have clearance for the chips. An upcut will throw out the chip and the chip can be bigger? at least for the rough cut. Do you have an ATC? Using the 1/4" in 1/4" steps is inefficient, try long ramping down then circulating around as well.

The width of the groove does matter as if its say 1/2" wide it can be done with a single tool if its 2" wide then you have to cut around it and manage the centre "plug" somehow... Peter