This is due to the low-pass nature of the IIR filter. With α=0.25, the passband (-3 dB) is at around 23 kHz, covering an input bandwidth up to audio frequencies (20 kHz). For tracking faster signals, ...

The cap in question is part of the guitar’s tone circuit, which along with a potentiometer forms a variable low-pass filter. A rich folklore has developed over the years around these circuits ...