I read books at different speeds all the time.

I read Mrs Frisby and the Rats of Nimh in a couple hours yesterday. But, I could only make it through around 30ish pages of War and Peace before my brain was tired.

I also read nonfiction a lot more slowly than regular fiction- though some literary fiction can be just as slow to read.

A lot of it is vocabulary familiarity or situational familiarity. If the characters are in a situation I've been in before or am familiar with, the reading is faster than if it's a completely foreign situation. It takes longer to conjure the image in my head if it's not a familiar image.

I try not to pay attention to page numbers while I'm reading because it takes me out of the book- but, yeah, sometimes I finish an hour of reading and I've knocked out 100 pages, sometimes just 20 pages if it's something really challenging.

All that is reading in my only language. I would imagine there'd be even more variation reading in a second language.