10 kHz, square wave. Slow, how do you mean? The QRV-04 has a lowpass filter at the input, R3, C5. This can be tuned to the value you like. If you have a pot this will have some influence. It is the total source impedance that counts.
The LP-filter is optional but I'll recommend it. You can decrease the 1 nF.
C6 may also be optional and/or tuned. The purpose of this cap is to force the feedback go directly from the opamp instead of via the buffer. This is more important when you use fast opamps.
If you have square wave as a test signal it's very important to compare the input signal with the output since you can have overshoots at the input (not perfectly matched impedance of the signal generator). Check also the oscilloscope probes.