You get the same BTUs regardless of the speed of the water through it. It will just not seem as hot to the touch. A black hose lying on the ground is a poor heat collector, for one thing its surface area to volume ratio is ineffecient. Small tubes with small volume but relatively large areas heat better, which is why commercial heaters use numdreds of short 3/8" tubes between large manifolds. A radiator works because it has "fins" attached to the water channels, but ONE radiator will be ineffective because the pool will radiate more heat away than it can deliver to the pool.