Well, English is my native tongue, but I have many friends who have English as a second language. One of them, a Korean, started out in the low 140s, but worked her way up to a 161. Considering the situation, I thought that was an admirable feat. There are two things I remember when asking them how they did it. One was that once they found there was a pattern among LG and LR questions, they were more easily able to predict the correct answer. Yes, all the books say this, but it's easier said than done. You mention that you've done about 50 prep tests. Go over some of them and look for some patterns among similar-type questions, if you haven't done so already. You may notice the the wrong answers normally tend to look the same: mistaken negation, mistaken reversal, etc. The second thing I remember them saying is that they initially had the same problem as you, they needed a dictionary because the test makers use so many obscure words. I would say this isn't a result of any kind of language barrier, because even native speakers don't know a lot of the words on the test. But even ESL speakers know the words surrounding the obscure word. Figure out the meaning in context and you won't need a dictionary for strange words.
One last thing. 5-7 wrong in each LR section isn't so bad. I don't know what you're aiming for, but it's easy to score in the 160s if you're only getting 12-13 wrong in the two LR sections. But you'd have to be able to get through the 4th RC section. Not sure what your memory's like, but a good memory can compensate for a less quicker reading speed.
Very good point. The bastards do know how to confuse even native-speakers with words like these.