I suspect that something about your test target, its placement relative to the cameras, OR the camera's tolerances for AF accuracy with their respective lenses, is causing the 6D to make minute focus adjustments nearly every shot, whereas the SL1 locks once and is happy with the focus distance for each successive shot.
Whether or not the camera tries to refocus comes down to how well locked onto the target it is, i.e. how confident the camera is that it has the focus correct based on its own internal programming and AF sensitivity thresholds and AF sensor size (area). When the cameras are configured to require focus lock before releasing the shutter, and AF-start is bound to the shutter release button (as opposed to the * or other button for back-button focus), both cameras will take a measurement of the target using their phase-detect AF point that is selected (if you have one manually chosen**) every time you press the shutter button. The measurement internally comes up with an error value, which is essentially how far off of critical focus the camera thinks it is. If it's above some threshold value, the camera will try to refocus, otherwise it will release the shutter. With a flat planar target that is highly contrasted and well lit, and the camera on a tripod such that camera-to-target distance is not changing even slightly, the 6D should attempt to refocus less often. Just having the cameras on a table and not using a remote release is prone to focus changes, as is having a 3-dimensional focus target (if that headphone cord is the target, that's pretty much a worse-case scenario--so many different depths in such a small area).
**If either camera is trying to auto-select the best AF point each time, this will introduce another delay.