C3 will object to rapid changes in base voltage applied to Q1. The capacitor is a filter of sorts I think, but from the perspective of the op-amp it appears to be to make an op-amp integrator circuit.
R4 and R6 are sampling resistors for the current sense resistor R5. Their values will control how quickly the op-amp integrator circuit around U1A responds and also their weight compared to pull-down resistor R7 and voltage divider/reference network R1, R2, ZD1. Omitted ZD1 is to provide voltage reference stability if input voltage has variance. R3 is apparently a 0 Ohm resistor, which surprises me as it drives a transistor base, but perhaps the op-amp achieves current control so a base resistor is unnecessary.
A switched mode current driver might be preferable and easier to use.
An op-amp integrator performs the mathematical operation of integration IIRC, but a more intuitive way to explain it would be it adjusts it output at a speed proportional to the difference in the error signal, so it adjusts faster when error is large.