Complex sentence: The large float and wing pontoons of the A6M2-N degraded its performance by about 20%, enough that the A6M2-N was not usually a match for even the first generation of Allied fighters.
Split into simpler sentences: The large float and wing pontoons of the A6M2-N degraded its performance by only about 20%. However, this caused the A6M2-N to be unable to confront the first generation of Allied fighters.