The BTU's will not increase. It's basically just physics. Your spa can only heat a certain number of degrees per hour and your spa will lose a certain number of degrees per hour.
Make sure, if you have air controls, they are all closed. If you can use the spa with only half the cover open, that would help as well.
If you can, turn your temp up to 105 or 106. You will get a bigger window to be in the temp you enjoy.