When you are looking at different pumps, there are a few variables to consider:
1) Inline or submersible. Any motor capable of moving a lot of water will have a fair amount of electrical resistance in the coils. This resistance means that some portion of the total wattage that the pump draws becomes wasted heat. With a submersible pump, all of this heat ends up in the water eventually. With an inline pump, some of the heat is transferred (by conduction) to the air in your room.
2) The total design of your system will also add some heat to your water. If you put the intake of your pump into a bucket and measure the time it takes just the pump to fill another bucket, you will see a flowrate that is very close to what your pump is rated at.
Now connect the whole rest of your system and try filling the bucket that way. Friction in your system will cause quite a signifigant drop in the rate at which water comes out the other end. The decrease in the velocity of the water is accompanied by an increase in the temperature.