I think you are operating on a level high enough that you're battling to breathe in rarefied atmosphere, progressing by paragraphs and sentences.

The example I gave was to "learn" Calculus as in a Technical School (Undergraduate level). The material on that level isn't, in my opinion, hard to read. What do you think?

Depends on how dense your source material is.

No matter your level of sophistication, if you are making it through ten pages a day, you should probably switch to harder material.

The specific number doesn't matter too much, though.

If you get through one page of dense math an hour, you are doing well.

I agree, with a caveat (see my answer to d-equivalence): In Undergraduate level for example, I think it would be useless to look for harder material for no other reason than the very fact it is hard. The person still has to learn the basics and that's the most important thing. Challenges are yet to come.

But, I agree that the material can be boring and too basic. This is why I have always preferred Soviet books (they get directly to the point and don't address stuff you should know) vs. US books (that will go back to high-school level to bring you up to speed, after which you want to sleep). Generalisation, of course.

Tried any classic French books?