We can imagine circular light as a combination of two rotating vectors moving through space: the electric field vector that´s rotating together in sync with the magnetic field vector. As the vector rotates doesn´t it take some velocity away from the propagating speed? In other words doesn't circular light propagate a very bit less than the speed of light?
Vector plots like this
should not be shown to students. Some falsely begin to believe, that the blue line actually represents something in real space. That is not true.
So what is shown here? This is, in fact, a snapshot of a single moment in time. I have arbitrarily chosen a convention (let me stress arbitrarily again), where I represent the electric field at a point by an arrow, which starts at that point and extends in the direction of the electric field to a length, proportional to the magnitude of the field. The proportionality between the length of the arrow and the magnitude of the field is again arbitrarily chosen by me. There is no physical significance behind it, thus there is no physical significance behind the blue line. The only motion of a photon is along the red line.
Still not convinced? I can freely change my convention and have the mid-point of every arrow placed at the point where I want to show the field. Then the above plot would transform to something like this:
Now the length of the blue line has changed, although nothing has changed about the distribution of the E-field. It's still the same wave. Same speed.
I can just as well adopt a convention, where the field at a given point is shown by an arrow, whose arrowhead is at that point, like so:
There is no longer a helix traced out. No spiral motion to take away from the speed of a photon.
The last plot is in no way better or worse than the first two. There exists no special rule, that the tail of the arrow absolutely must start at the point, at which we wish to represent the field. The 2nd and 3rd plots are just as valid.
Finally, here's how a propagating circular-polarized wave looks like:
It is, essentially, moving as a whole, so again, there isn't some spinning motion to take away from the speed of the wavefront.
Circularly polarised light can be thought of as the sum of two linearly polarised beams which are at right angles to one another and ninety degrees out of phase.
The linearly polarised beams components travel as the same speed as each other and their sum.