  We report on Mid-infrared (MIR) OCT at 4 $\mu$m based on collinear
sum-frequency upconversion and promote the A-scan scan rate to 3 kHz. We
demonstrate the increased imaging speed for two spectral realizations, one
providing an axial resolution of 8.6 $\mu$m, and one providing a record axial
resolution of 5.8 $\mu$m. Image performance is evaluated by sub-surface
micro-mapping of a plastic glove and real-time monitoring of CO$_2$ in parallel
with OCT imaging.
