The best way to improve timing on RC and CR is to read at a deliberate pace. Not too fast, not too slow. If you read too slow, you will easily go over time. However, if you read too fast you will waste time re-reading. Therefore you need to find an intermediate pace in which you can comprehend the entire text. Note that for CR, it is important to read all the details - since a small word or phrase can be the key between two answers but for RC the big idea is more important (don't waste time reading all the details of the study provided, at least not on the first go). You should also practise reading outside of the GMAT questions - pick a publication like Financial Times, Economist, etc and read articles from it, at the same deliberate pace, so you can practice reading. Make sure you understand the article too - try to summarise it in a paragraph once finished and see if you have actually understood what the article is saying or not.