Average life expectancy for someone with Alzheimer's is 5.8 years [0], so Shannon's couldn't really have been quickly progressing unless he was diagnosed extremely early.

I remember when my mom got it, I did a bunch of research and figured out that she probably only had about 3 more years of okayish memory and a decent quality of life, and then maybe 5 more years after that until death. That stuff depends on age and gender, and my mom was young so I think those numbers are even shorter for someone who is old and male like Shannon. There is of course plenty of variability and some people live decades with it.

So yeah, doesn't sound like "very quick" progression. But if your baseline is a healthy adult aging normally, it would still seem very quick compared to that, I guess.

[0] https://www.healthline.com/health/alzheimers/life-expectancy