my IS feels sluggish

ive noticed that in the last week ot two my IS300 feels sluggish and has a hard time getting up to speed or growls a lot more when i floor it... its been getting a little warmer and arid so im guessing it may be because of the engine sucking in hot-dry air instead of cool-dense air... also i had my front two tires changed... i think that may be a factor but doubt it...

has anybody felt a drop in power or does your IS300 seem a little less eager after a while ?