PhD supervisor is taking really long to read my thesis
A good starting assumption is that your supervisor has good intentions, but like many of us, has trouble getting things done because of workload and/or procrastination.

Artificial deadlines may help.  Perhaps your supervisor is too busy right now but would be able to agree on a date several weeks in the future by which he can read the chapter.  Or even better, perhaps you could agree an overall schedule for completing your thesis, including deadlines by which you will send each chapter, he will review it, and so on.  This could help prevent the problem from recurring in the future.

If this doesn't work, I would have a chat and share your concerns with him about how you feel this is needed for your own successful completion of your thesis.  

If all of this still doesn't have any impact, you are in a tough position.  You are dependent on your supervisor, but outside of extreme situations, there is no real formal mechanism requiring his action.  You could try getting your second supervisor involved.  Even if you have had no contact so far, you do have a formal relationship with this person, and that is a good enough basis to do so.  

Talking to the director of studies would be a last resort.  It probably isn't worthwhile until the continued inaction is clearly going to delay your PhD completion.  If you want to maintain a good relationship, I would discuss this with your supervisor first, making it clear that the delays are about to have a big impact on your life, and you feel you have no other choice.