Yes, but it perhaps more instructive to think of a curved mirror. In case of a raster engine, you need to calculate a texture for every triangle of the mirror. ( Which is essentially the same as rendering the scene in the first place.) On the other hand for a ray tracing engine you just need to reflect one ray on the mirror surface for every pixel the mirror occupies in the final rendering. So compared to the same scene with a window instead of the mirror, you just need to calculate one additional reflection instead of running the entire rendering pipeline again.
Yes, I was essentially using one ray per pixel as shorthand for constant amount of rays per pixel. (As in 'I did not think about that point.') One needs more than one to avoid creating artifacts like aliasing ( see [1] for a very nice example), or depending on how 'naive' or complex the algorithm is for stuff like shadows or caustics. But for nice scenes (which do not create artifacts) with mirrors and lenses, one ray per pixel would work fine.
