Some time ago, I read a study done that compared different slide films. I think it was done by Wilhelm Research. If I correctly recall, while Kodachrome has a very long archive life if kept in the dark, it tended to fade faster than some of the E-6 films when projected.

However, nearly all of our family slides taken in the late 1950's through the 1960's, and even newer were shot on Kodachrome and projected. No doubt some of those were projected many times with some of those being viewed in the projector for extended periods while stories were being told. For the most part, they seem to have held up very well. But of course I am relying on memory as to how they looked years ago and even my memory can fade over time, so I can offer no scientific proof.