There is definitely qualitative and quantitative difference between upper case and lower case reading experience.
This has to do with reading speed, familiarity, shape and eye movement. I myself am a fast reader (moderately: I've been clocked at bit over 1200 words per minute), so most of what's below is from experience.
Lower case is simply smaller, so I can take more words with a glance (again, I've been tested with capacity of reading 3-4 words at once, depending on their length).
Basically, reading at any sensible speed requires reading more than one letter at once. Usually it is at least a word, but speed read record holders can read whole page at once.
Since I read a lot, also aloud, interpunction is important, and while not covered by name, upper case is part of it.
Reading a lot also conditions brain to the accepted upper/lower case use.
So adding all that together you get all sorts of difficulties while using upper case. Small upper has the advantage of being, well... small, but that's about it.
Miles Tinker also mentions it:
Text in all capitals covers about 35 percent more printing surface than the same material set in lower case. This would tend to increase the reading time. When this is combined with the difficulty in reading words in all-capital letters as units, the hindrance to rapid reading becomes marked. In the eye-movement study by Tinker and Patterson, the principal difference in oculomotor patterns between lower case and all capitals was the very large increase in number of fixation pauses for reading the all-capital print.
If I remember correctly from an TV show of some kind on the development of Transport font for UK Motorways, during that development all kinds of fonts, variants and ways were tested and they came away with results that lower case was superior by a large margin. I think they specifically mentioned testing the smallcap variant as well, but it still was problematic.
Just FYI, I find reading ALL CAPS physically painful. No joke here. If I see on the internet something written in ALL CAPS I simply skip it (I mean here whole sentences, posts or messages). I'm not shy in using it when writing, but try to limit myself to as few ALL CAP words as possible.