Actually, those speeds at those times aren't that bad. It would be best to set up an account at testmy.net then run a few tests at various times of the day, if possible. Again, though, the speeds you are getting at those times aren't bad.
15000 to 20000? I don't know your plan, but I imagine it states speeds up to 10MB. UP TO are the key words, and again, those speeds that you stated aren't bad for those times. Again, you should set up an account at testmy.net so it can record your tests and the speeds of said tests.