it's not a specific Source Audio or C4 issue, you will see patch change latency with most powerful digital pedals. I see the same situation for Strymon, Eventide, EHX. Unlooping a pedal with a looper / switcher...that's fast and can be done during a song without any latency risk. Changing patches mid song...that's more tricky and I tend to change patches between songs only. That's the physics of digital pedals. Hey...still quicker than a manual dial in across 18 analogue pedals.
Ok thanks - I suspected that was probably the case and as you say, the power of being able to make simultaneous changes to so many pedals outweighs the drawbacks for me. Just wanted to double check I wasn't doing something stupid!