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Two words: oppressive humidity. Pretty much anywhere in China you will experience heat and humidity that will make you crave AC. Some good advice is get your stuff done early, beat the traffic and go home to chill or hang out in Starbucks and get some work done. 
I find I have zero appetite in the summer so mostly eat fruit. Fruit is cheap and good and I hate cooking when its this hot.
Smog seems worse in the summer so another good reason to stay indoors. I just go to the gym, Starbucks, buy fruit, read and watch YouTube. Beer with friends tastes especially good on hot days so a good excuse to have a few ice cold ones.

After 19 years in China, it's still difficult to adapt to the summer climate - excessive humidity, low cloud formations trap the heat to make it oppressive.  We also still have smog from the many factories that belch pollution from coal-generated plants in Henan and Hebei provinces.

Best is to remain indoors. If you must go out, buy heat-reducing remedies from local pharmacies, such as a vitamin face mist - designed to keep your face cool on a sweltering day;  a Chinese concoction called, Huoxiang Zhengqi Shui - it promised to cure everything from sunstroke to dehydration.

We also have myriad coffee shops where one can plug in a computer, sip  fruit drinks and relax.

Enjoy your summer.  Don't forget sunscreen and yes, buy a fan, either battery powered or a regular hand painted Chinese style fan.

David

Xiamen has great weather in Summer, and great air quality. But: it's HOT and HUMID.  30° to 35°C  at 80+% humidity are normal here, but it feels more like 45° to 50°C. If I do any sports activity in summer then it's usually before 7am, or after 7pm. When I go to have a walk outside I always follow the shadows and hope for some clouds.

For the rest of the year  the temperature is very comfortable but the air is not as good as in Summer. And there is a lot of rain in Spring.

I have lived in China for eighteen years.  Primarily in Guangdong province and now in Guiyang.  I never had much of a problem until I moved to Guiyang.
Most days I feel bad.  Seldom go outside except to the gym and that is a major struggle to walk the two km each way.
Perhaps, it is the air quality, the elevation or both.  It does rain a lot here and the temperature varies.
I often feel uncomfortable, more often than not.