Actually, the rectification itself causes harmonics independent of switching speeds. The latter are responsible for really high frequency (think RF) currents.
If you take any periodic wave form that is not sinusoidal (and the rectifier current is not sinusoidal whether you use a capacitor input or a choke input) you can represent that waveform as the sum of a fundamental sinusoid and higher order sine and cosine harmonics of that frequency.
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