I make my own chains. Most of the chain stores carry the harry homeowner "safety" chain. You want a tooth with the chisel point, its got a 90° to it instead of being rounded. Also as mentioned, the rakers have a lot to do with cutting speed.
If your chain has been sharpened a few times you may need to cut the rakers down. If they are sitting too high it wont cut well.
I have a 455 rancher and it will handle the 24" no problem. I usually run a 20 but throw the 24 on when I need it.