I would say , relative to the outer two parts , it is easier. I am well aware that there are slower and note-wise simpler compositions that may have musical/interpretive difficulties and thus can be more difficult than the faster running parts of the composition. But in this case, the fantasie impromptu, yes, the middle section has its difficulties in interpretation musically that one has to expound upon. However, the faster outer sections has its own musical interpretation difficulties that are about par with the middle , if not more.
There are pieces like op 10 n 4 that musically it is not too hard to interpret as long as you are playing fast, accurate , and in rhythm !