improving smps emc conducted spike
possible general source of problem is the switching element?
Surely true somehow, but it's actually a tautology. The problem is to identify the pathes, that emit the switched voltage and currents, obviously present in any SMPS, to the enviroment.
More precisely, consider a square wave as switcher waveform. It has a lot of harmonics. If you see a broadband series of harmonics, at best of decreasing magnitude over frequency, it's normal operation. Increased magnitudes at some higher frequency bands or peaks indicate resonances and should be interrogated more thorougly. Also some switching elements, e.g. schottky diodes can be expected to generate more high frequency harmonics, that need specific filtering or can be possibly reduced by using a different device. Non harmonic peaks may indicate parasitic oscillations of a switching transistor.
In you circuit, you apparently have nearly
no effective EMC filtering, so it's quite normal, that part of the switching energy is spreaded all over.
A moderate reduction of switching frequency won't help much, I guess. A large reduction requires redesign of the magnetics, possibly causing additional interference problems. Please consider also, that in principle, higher frequency interferences can be filtered more easily.