I recently asked this question to my
magoosh instructor. And he answered it beautifully. Hope this will be helpful for others too.
As far as speed goes, this will increase with general proficiency. I'd also suggest that you read high-level material every day. The faster you can read, the faster you can solve these problems, as there's a lot to read and consider!
Further, your eyes and your brain can sometimes work at different speeds. For instance, when reading really tough material, I can "read" the words very quickly, but I have to slow way down in order to understand them. Conversely, with easier material, my brain processes the sentence before I'm even done reading it, and I'm already thinking of connections and counter-arguments before I've even finished the reading.
So not only do you have to work on reading words quicker, but you also have to work on your brain's ability to make sense of them. Just like any other muscle, your brain needs to be worked out. Of course, GMAT prep itself it a very good workout, but so can more interesting reading material, a stimulating conversation about some abstract topic, or even a difficult puzzle or board game.
The trick is to push yourself. Focus as hard as you can, and when you get tired and want to stop, focus even harder! You want to pay attention to whatever you are doing for longer than you want to. That's how your brain will get stronger and quicker.