How do parallel resistors and capacitors reduces frequency peaking in opamps?
Reason for C15

Without C15, parasitic input capacitance on pin 6 to ground would causes a peak in the amplifier response because the negative feedback at high frequencies gets less and therefore the resulting gain gets higher. Consider this: -



In effect the impedance "R2" is lowered by parasitic capacitive reactance so you get more gain at higher frequencies. You also get more noise (it's called noise gain) so C15 reduces this too.

Reason for C20

Parasitic capacitance to ground on pin 5 will attenuate the input signal passing through R20 so, C20 is there to counter that effect.