From a theoretical point I believe it depends on where you measure it. From a practical point, if 1 cfm is supplied at the surface compressor it will again be 1 cfm when the bubbles reach the surface.
But back to the theoretical, as the air is compressed it reduces the area it is contained in. So if there were a flow meter at depth, it would read a lower cfm reading, because the air is more dense.
Eons ago we used to use a balloon for demonstration purposes scuba diving in the swimming pool or lake. The instructor would take the balloon to depth, partially fill it from an attachment hose coming out of the first stage of his regulator, then release the balloon. If filled just right or if at great enough depth, the partially filled balloon would burst before reaching the surface. The object lesson for divers was that if you breathe compressed air at depth, then hold your breath while assending, your lungs would do the same thing. Not a good thing.
So as the air becomes denser with depth, the pressure has to increase to keep it moving deeper and the volume it displaces becomes less, thus a lower flow at that depth measured in cfm. But once the air is released from its pressure it expands back out to original.
So from a practical standpoint, 1 cfm at the surface that the pump puts out is still 1 cfm when the air reaches the surface in the form of bubbles. But at it compresses the volume decreases as the density increases and the velocity slows. That is why it is more important to have a big line near the compressor and a smaller one is ok nearer the diffuser on very long hose runs to deep water.
On my diving regulator the hoses from the first stage (on the tank) to the second stage (at my mouth) is very small. Probably a quarter inch in diameter. If I were to try and breathe atmospheric pressure air (as in using a snorkel) through this quarter inch diameter 2-3' long hose the breathing would be difficult at best. Like breathing through a small diameter straw. Yet at 125 psi the first stage delivers to the second stage, there is plenty of air because the dense air in the hose expands as it drops in pressure to my breathing pressure (atmospheric pressure at the surface, not quite 15 psi at one atmosphere or 33 feet depth, not quite 30 psi at 66 ft depth and so on). It is hard to imagine but when I'm at 135' depth if I were breathing from a hose supplying air from the surface, the air pressure I am breathing and the compressor would need to supply is just under 60 psi. But since the surrounding water column pressure is the same, my lungs see it no different than breathing atmospheric pressure at the surface. But I dare not hold my breath as ascending - because as I go towards the surface my lungs are trying to hold in 60 psi air.
Not good.
More than anyone wanted to know.