I've noticed that my Charge runs much better n smoother below 105 F and the battery life is also longer. So what I would sugest that you take off the back from the phone and put an aluminum plate a 2x2in to the battery and then put the deffender box on. The alluminum plate will draw the heat faster from the battery keeping it cooler therefore linger battery life and better performance. I personaly have the - traveler - which protects only the back and the sides but I also have the battery cover off so there is some room for the heat to dissipate faster n easyer than with the cover on.
Its what I would do if I had a deffender. Heat is not good for the battery or the phone that is why its so lagie, and then faster when it is out of the box.
Good luck.