I appended a conclusion to my original reply from last night. Stephen, thank you for the answer. It appears relativistic velocities like the muon are not as common as I imagined but even these lesser velocities you mention would accumulate into time dilation like the protracted decay of the muon just on a smaller scale. Normally this dilation is intangible as the atoms must exist in a different inertial frame only briefly passing our frame of observation. If we can accept time dilation due to mass acceleration relative to the time axis then we should also allow for the opposite case of a moving time frame relative to a stationary mass. I think this is already the case for equivalent acceleration when a crushproof spacecraft sits on a collapsed star but instead of inhibiting the flow of time like the dead star, the Casimir cavity takes that same sort of inhibition (accumulated pressure) and exhausts it out of a tiny cavity too small to exhaust the reservoir. This creates a steady stream many times faster than the isotropic field it is breaching. My point is this represents a difference in acceleration between inhabitants of the cavity vs outside the cavity such that "velocity" accumulates -I put velocity in quotes since from our perspective it is time dilation but from the local perspective of the gas atoms inside the cavity it is feeling equivalent acceleration and sees the cavity walls shrinking away into the distance even though its spatial coordinates are unchanged. This makes time dilation tangible in that We can use gas atoms in a stationary catalyst/reactor to exploit this environment.
In conclusion the temporal "rate" of the Casimir cavity by virtue of its vector should be considered immediately "relativistic". It doesn't have to achieve a right angle relationship on the scale of C because it is directly altering the time axis. Relative to the isotropic time flow outside the cavity there is an absolute difference in "equivalent" acceleration meaning that gas atoms diffusing between the 2 frames will experience abrupt changes in time and gravity we refer to as catalytic action (change in Casimir force). Best Regards Fran