Speed of baking fluid simulation slows down drastically When baking a fluid simulation the process slows down.

The simulation has object of type inflow and object of type control.

The speed of baking will go down from rate 1 frame/min to 0.06 frame/min within 50 frames.

Is this typical behaviour?

Time is a resource and if this is typically true then it helps to plan time allocation.  If there are methods to increase the rate then that is useful.
Using Fluid Control objects can significantly affect your bake times - particularly when using a large Radius in the control object.

The control object 'radius' determines how close fluid needs to get to the control object before it is influenced by it. When fluid is being influenced by a control object the fluid simulation has to do a lot more work to determine how that fluid is affected by the control object (which effectively emits a force on the fluid). Therefore, the more fluid is within the radius of the control object, the longer the simulation will take.

This means that at the start of the animation the baking may take a small amount of time but as fluid gets closer to the control objects it will drastically slow down.

To limit the overhead of control objects, keep the Radius of effect as small as possible for your purposes. If you are only using stationary control objects you may consider reducing the Velocity Force radius to zero.