I heard some reports that Straight Talk slows down your data sometime after just a month of service. I have also heard that they slow down data and then speed it back up every few weeks. I have experienced this to and it seems that they are still giving you the same amount of data. The catch is that instead of giving you say 3-4 mbs download they change it to 0.50 mbs download and move the faster speeds to upload. I noticed when I was getting very slow speeds I was getting .50 down and 5 up. I don't know how they do this but next time your data slows down on Straight Talk do a Speedtest and check the upload speeds.
If Straight Talk slows your data speeds down then just a quick call to customer service usually fixes it (for a little while).