Their long S is really annoying, although truthfully I generally am unfamiliar with the long s in modern fonts so I don't KNOW if it really looks worse than it needs to, but I feel it looks worse that it needs to and that makes it harder, for example I thought lest at first was left and had to go back a couple words after.
Anyway as I know from my reading history at 1400 it gets difficult, but I can make it through 1400 and 1300 with difficulty, but would need to break out the middle English dictionaries for 1200 and 1100. 1000 forget it, too busy to make that effort.
Interestingly I found the long s annoying and I had to think every time I saw it, but I quickly got used to and could read it naturally after a few paragraphs.