getorf - it's really cool actually. I've not kept up with the research, but about four/five years ago, someone was publishing about it all.
Basically, you know how vertical black-and-white stripes can bother some people with epilepsy and similar, especially if they're moving their eyes across them? Well, they do.
Actually, they bother most of us to a greater or lesser degree, it's just most of us don't really notice too much.
Now some fonts are made up of shapes such that, when you're looking at them out of the corner of your eye, they're resolving into a set of shadowy stripes. You don't even realize this - when we read, we do a huge number of things we don't know we're doing, as you can see by tracking someone's eye movements (people pause and look ahead or back, when to all intents and purposes they're reading perfectly smoothly, because the pauses and glances only take such a tiny fraction of time).
As you read a large amount of a font that has a pronounced, regular vertical emphasis, you struggle against this slight visual disturbance.
People read such fonts more slowly in tests.
What is really fascinating is, they thought they were reading perfectly well, even quickly, and saw the regularity of the font as something good.
When tested against a font designed (like script) to have more irregularity and less vertical emphasis, they read faster, but didn't believe it - they associated the irregularity as something that would disturb them.
I trace it back to (proper, real) gothic script developed in monasteries. I won't go into why, but basically, back then in made sense, because you wanted people to read very slowly. Now, we don't, but we're still stuck with something not dissimilar to scripts developed 500 years ago, really.