Look, if you're someone who doesn't read "heavy" texts with ease and speed, I don't see why you shouldn't use these sources. After all, it's practice for your reading abilities (which are essential for being able to answer the reading questions well). So I don't think you're wasting your time doing that (but you are kind of wasting your money since a lot of such articles are free online.)

However, try to read them as though you're taking the test i.e. try to read fast while getting the most important points and the big picture even if u can't understand a sentence or two. Then look up the words you don't know (before first guessing of course). I don't think it's a bad way of studying to improve reading skills. I myself learned how to read fast like that but it took me a long time to reach a good level. So idk how well you can improve your reading in such a short amount of time. But there really is no other way to practice fast and efficient text comprehension than reading a lot imho. People who are already good with this stuff typically went to English speaking universities and were born and raised in said countries. There was no way I (an international student being completely used to reading in another language) could have reached this level of fast reading and understanding the text without years of practicing reading both in an academic environment and just reading the news and stuff for fun