They don't if you have two hands!

Wait, what?

Per letter, capitalization doesn't take much longer, but over the course of an entire sentence or paragraph, having to reach for shift can potentially add up. Sure, it's still small overall, but I think it's reasonable to give the benefit of the doubt to a stranger about whether the extra effort for them to type them is more than the amount of extra effort for you to read it.

It doesn't take longer for me as I have both of my hands on my keyboard and I can hold the Shift without any delay (or noticeable delay), simultaneously. You could even use "caps lock" in which case you wouldn't have to use shift at all.

The shift key is an uncomfortable stretch for my fingers. I can do it, but it isn't comfortable.