The thing about the rapid rooters is they stay too wet. When I first started using them, the clones would rot before they ever rooted. So I keep the tray dry and just wet the plugs down before I put the clone in it. By the time the plug is dry they have rooted. Since I started keeping the bottom tray dry, I have lost 1 clone. And when the tray is full. I need to open the dome twice a day and shake the drops off the dome. It's just too humid. So maybe the trick is to keep them a little dryer then you think you should ?
Havent tried rockwoll cubes. I should just to see what kind of luck I get with them to compair. And that 4 day one was just lucky. It normaly takes about a week. I put some clones just in soil under the dome. The plugs rooted about 3 days faster. After about a week all of the rapid rooters were rooted and only two of the six soil ones were rooted. So I put the soil ones in the rapid rooters and ended up with roots on all of them in about two more days.
So I can safely say that the rapid rooters are alot faster then the basic soil rooting I used to do. But I cant say it's faster then anything else cause I havent tryed it, Sorry bro.