Chilling wort is more complicated than it may appear on first glance. Some of the big factors have been covered in this thread already. Temperature differential between your wort and the chilling water in the chiller is huge. Generally: the more the faster, but it's not that simple. Agitation (stirring) is important, and a moment's reflection shows why. The chiller is constantly moving the temperature of the wort just on the other side of the chiller wall down, thus decreasing the differential. So- stirring a lot is really helpful, because it's constantly mixing and thus raising the temperature of that wort right next to the chiller wall, thus increasing the efficiency of the process.
Of course, the material of the chiller is important. A copper chiller has better conductivity than one made of stainless steel.
There are also things that go on INSIDE the chiller, as I learned when I went to change out the connectors on my Midwest copper chiller (just the entry level 25' 3/8" tubing model. I got tired of the hose clamps that held on the vinyl hoses leaking, so I changed them out to compression / hose fittings. While doing this, I noticed that there is a yellow plastic tape in the tubing that runs the entire length of the chiller. I e-mailed Midwest about this, and they said the plastic tape is put in the tubing on purpose, to increase the efficiency of the chiller. While it does retard water flow to some extent, it mixes the water in the tubing, thus apparently accomplishing the same thing inside the tubing that stirring does outside. The Midwest guy said that without the tape, it would take 15% longer to cool the wort. Interesting.
My cooler will take a full 5 gal. boil to 70F in 21-22 minutes, but we have a well, and the water is pretty much 55F year-round.
If your water is as warm as you say, perhaps you'd benefit from going to the pond pump / ice bath system mentioned earlier. I have a submersible pump, and have considered doing a recirculation into a large cistern on the side of the house, that usually is half full with probably 1,000 gallons of ground temp water in there. Same principle.