Hard to read
One of the members of my Ecom Convert group shared some interesting research about ALL CAPS text in a comment to a mini-teardown last week:
Roughly 90% of people find all-capped text to be harder to read, in general.
These two articles were given as sources:
I dug a bit deeper
Glanceable text on your store can be, for example, labels on product thumbnails in your product lists.
In that same article by NN/g, they also confirm that for longer text (titles, subheadings or paragraphs, for example), ALL CAPS isn’t a good idea.
Now what the studies don’t mention is whether slower reading is always a bad thing. I could imagine why having users slow down and take more time to read a heading isn’t such a bad thing. I have yet to find any study about that, however.