Why do keyframed objects cause fluid baking to slow down so much? In the following figures we show the results of our attempts in order to mix a key-framed animation with a fluid simulation. As you can see, in the top one the obstacle is static (fix) but in the bottom one it turns counterclockwise slowly.

The Static one took only 5 minutes to be baked, however, the dynamic one took 65 minutes on the same computer system!? 

We need to know:
1. First, whether this massive time difference is usual even though there is no significant difference between the two simulation?
2. Is there any tip / trick to speed-up the baking of mixed animation/simulations?

Our final simulation (for education purpose, btw) will be much more complex in terms of dynamic and the number of objects in it.
Yes. The liquid system must translate one moving object to the domain matrix, making it process for every frame, and calculate the new differences and forces produced by the push of the object over the liquid. Of course, this requires more time. 
I guess if the obstacle is really complex (more faces), this also requires more time.
If you can use lower resolution, less to time to bake. Use lower resolution for tests and increase the resolution for the final product. Take a look at my answer on your other question.