Don't think sled dragging is a good idea. It would be better for developing muscular endurance than it would for speed. It's one of those things like putting on ankle weights that sounds good but doesn't work in reality. It comes back to moving slow to train speed. The things you do to train need to be closer in nature to the thing you're training for.
Moving slow with a sled reinforces moving slow. Also running is VERY form specific. If you've ever run track you'll know that you have to get down your technique and improve that to go you fastest. Dragging something changes the mehanics too much. Puts you in a position conducive to dragging NOT sprinting. You would get MUCH more benefit out of regular sprint drills without a sled.
I think sled dragging would be great for GPP. Great for improving work capacity. Strengthening the posterior chain. Great if your a strongman
. But look at how you drag things as opposed to what you want to do to run fast. To drag something you STAY DOWN LOW. If you are in a sprint race the key is to reverse off the block and GET UP HIGH as quick as possible. Staying down low and moving slower = a bad way to get faster. That said it's probably a great supplemental thing for here and there. Just like strength training. But it should not be a main component of "speed" training.