ALL cars lose power in hot weather due to the lower density of the air, if you are that bothered you could put water sprays on your intercooler, that will give you a bit back.
But there's also the climate control, if you have it on auto it will be using the compressor almost continuously to cool the interior in this weather absobing some engine power [There was a thread about this some time ago] If you don't mind sweating your b*ll***s off in the car turn it off and you should feel some difference.