This is a tough one - you can improve, but it's going to take some time and concerted effort.
Essentially, for someone who is a non-native speaker and who never spent much time reading in English at an academic level, what you need to do is take a step back from studying how to do this for the GMAT. The GMAT requires you to read at that level
quickly
. You need to learn how to read at an academic level
in the first place
. Then you can learn how to do so more quickly.
The good news is that this will help you when you get to business school, too, since you will be reading a large volume of academic material on a daily basis.
So, set aside your RC and CR studies for about 2-3 months (you can continue with your other studies) and start reading. You're going to be reading all kinds of things:
Academic:
http://magazine.uchicago.edu/http://harvardmagazine.com/Business and Science: Wall Street Journal, New York Times, The Economist, Scientific American. Look for longer / feature articles, especially in the week-end "Magazine" section for the first two.
If you can, find a friend who's a native speaker and strong reader and who's willing to help. Once or twice a week, read the same article. Then, give your friend a summary of the main messages of the article (sometimes do this on the phone or over coffee, sometimes do this via email). Ask your friend to critique your understanding - point out messages you may have misinterpreted or missed entirely and show you how s/he knew what this message really was (literally, but quoting from the article!).
Also, start reading novels and non-fiction! I'm a fan of the authors Doris Lessing (fiction), John McPhee (non-fiction), Pearl Buck (fiction), William Manchester (non-fiction), Thornton Wilder (fiction). I also love Agatha Christie (mystery novelist) - the books are on the shorter side and a bit "lighter," but she's always grammatically correct and on the somewhat more formal side, writing-wise, so it's good practice for language skills. Ask your friends who love to read to recommend other books.
When you start to feel more comfortable doing this type of reading in the first place, then you can start to work on reading more quickly.