REW - Umik 2 (timing drift)

[OCA]

You could place your mic a metre or so from a speaker and pointing directly at it and make some measurements to see if the variation is mainly due to replay chain clock variations (if the variation is still there) or environmental reflections at the measurement position (if the variation goes away). It's all probably academic as far as your imaging concern goes however, since the total variation in the measurements you posted corresponds to less than half an inch of movement. If there aren't gross arrival time differences between left and right at your listening position any imaging issues are more likely to be due to differences between the left and right channel frequency responses.

@JohnPM While you are at it, is it safe to assume clock correction option will be useful even when a USB mic has an internal clock like the Umik-2 because it deals with clock differences between the input and output?

 

[JohnPM]

@JohnPM While you are at it, is it safe to assume clock correction option will be useful even when a USB mic has an internal clock like the Umik-2 because it deals with clock differences between the input and output?

Yes, clock correction exists to cope with differences between record and replay clocks.