Not only can it be so perceived despite such slower pace of travel, it always is! Right now, all the light you are seeing is light that is traveling slower than $c$.
Here's the thing: Your eye has, within the eyeball and filling up the entire space between the lens and retina (the "detector"), a gel-like material called the vitreous humour:
(Adapted from: https://en.wikipedia.org/wiki/Vitreous_body#/media/File:Schematic_diagram_of_the_human_eye_en.svg. Courtesy Rhcastilhos and Jmarchm. cc by-sa 3.0.)
Mostly this is water, but also contains a protein component which makes it thicker and more like a gel. In any case, the refractive index of this medium is strictly larger than 1: according to this source , it is about 1.337. That means that light is traveling about 75% as fast as in vacuum (roughly 225 Mm/s instead of 300 Mm/s). Moreover, in all other components of the eye, similar laws apply: the lens, through which light must pass before it gets to the internal fluid, also has a refractive index greater than 1, as it must in order to function as a lens.
Moreover, photons of light are not absorbed at the retinal surface, but instead pass through the surface to reach sensitive molecules in the cell bodies underneath. All this intermediate material also will have positive refractive index.
Hence at virtually no point - from entry to the eyeball to final absorption by a molecule of visual pigment (the stuff that the retinal cells use to register light, effectively a form of photon counter), is the light traveling with the vacuum speed of light ($c$). And yet, this light registers all the time, or you would not be able to see!
Regarding your other questions about the physics of speed changes, the answer is that the speed of an electromagnetic wave is set by the medium in which it is traveling right now. It is not set by the past history of the waves, which is what it seems you're referencing with the prism: that when it exits, it will still be going slow, because it was "slowed" therein. Instead, light always travels at $c$ in vacuum, and at some speed less than $c$ in the prism. When it is approaching the prism, since it is in vacuum (well, on Earth, air, but we'll just use this to make it easy and moreover the same principles still apply), then it will be traveling at speed $c$, since that medium is setting the speed to that value. When the waves begin to traverse the prism, that medium sets them to have a speed lower than $c$. Finally, when they reach vacuum again, since they are in vacuum now, it resets their speed to $c$ again. Then, of course, when they enter your eye, they drop below $c$ once more for the final detection.