1) Got you. But is speed improvement really there? :) It's one instanceof vs one (or two) get() + at least one if.  Plus to my understanding those operations are almost free in comparison to the cost of logic applied inside transformations.

2) Do we have any tests to measure the speed of JOLT?

3) Also, a bit more general question. How concerned about performance are we? I usually try to not trade good design for performance gains since well designed app can usually be greatly optimized by JVM itself.