First and foremost, Ivanhoe is entertainment – it's not supposed to be a brainteaser. It's the 19th century equivalent of beach reading. Think The Help or The Hunger Games and you'll be at about the right speed in terms of intended difficulty.
But this is entertainment circa 1819, almost 200 years ago. The language and style of English has changed a lot since then, and Ivanhoe sounds rather formal by today's standards. What's more, Scott occasionally mixes in even older language – like "thou" and "thy" when "you" and "your" would do just as well – to create a sense that Ivanhoe is more ancient than it really is. (People weren't still using these words in 1819.) This mixture of 19th-century and faux-medieval writing makes Ivanhoe a bit more challenging than your average modern page-turner.