I think sometimes we feel we ought to read something but we just aren't finding it interesting enough to hold our attention. You can clearly read quickly and easily when it's something you're really enjoying.

One of the reasons you might have problem with Lenin and marx's writings could be because of which version you read. I myself didn't have that much trouble because i read the ones hosted on prolewiki (currently down unfortunatly, I have some works downloaded if you want) which have additional notes on people and events the books themselves don't go into. I'm not sure if English is your first language but if it isn't, to my experience, translated versions also do a good job of giving additional information about those. So you could also try those instead.

But yeah, origin of the family is just a weird read. I didn't have any problem while reading Marx and Lenin but origin of the family was just annoying and somewhat boring to read.

And as other comrades have said you kind of need to get used to it. These authors lived a hundred years (and more) before us and language changes a lot within that time. It's only natural to not feel instantly connected to their way of writing. As you read more it will become normal to you.

I found it helpful to practice reading by reading fiction in order to build up the tolerance for non-fiction. Short Stories also help build up a tolerance for reading longer books.

Theory can be dense and the unusual language can make it more difficult to get through. What you are describing is very normal until you have gotten used to it. You can try reading something more casual like a fantasy novel to relax in between reading theory.

+1 for audiobooks and if you're like me you'll want a mechanical task you don't have to pay attention to

I’d say that’s probably not uncommon. Try audiobooks that’s how I read a few that were just too dense for me. There’s also like a study guide/student version of Marx’s Capital that was really helpful. Luna Oi helped publish or write (both?) a book on dialectical materialism and those were easier to understand than some of the older theory books. There’s also nothing wrong with taking breaks. I stopped reading outright communist books for a bit to read parapolitical books and now I want to go back to theory.