The reason speed and pace aren't always interchangeable is that speed is fundamentally a more abstract noun.
Fast is an adjective for an object's quickness of motion. A pace can be a step or an object's rate of movement (in other words, a speed). But while speed can also be a rate of movement, in its more general sense it refers to an abstract concept that is a scalar (continuously variable by an attached magnitude) quantity of motion. For example, one pace can be faster than another, and one speed can be faster than another, but an object cannot have more or less pace as it could more or less speed.
So when the sentence says, "…at considerable speed," it refers to an object moving at significant [amounts of our abstract concept that is speed]. But that does not equivalently mean the object is moving at significant [amounts of pace], because "pace" already refers to a particular amount of speed, and not to the general concept the quantity is. However, if the sentence had a determiner such as a in "…at [a] considerable speed," speed and pace would be interchangeable, because "considerable" would then be describing an attribute of a particular speed or pace, and not the extent or degree of the concept present.