Peaks in XOR-XNOR Waveforms
The pulses you are seeing as "peaks when both inputs transition" is totally due to different time delays through the circuit the from either input to the output. 

Logic chip manufacturers go to great lengths to design circuits in an attempt to equalize the logic delays through the circuit so as to minimize the effect that you are seeing. Invariably this leads to a circuit with more transistors and careful tuning of the various device sizes (channel widths and lengths) to get the logic paths more in line with each other.