it has been the same when ambient temperature is high the phone simply cant handle the heat and u can have marshmalow or oreo no mather what it wont help...phone starts to lag to a point when u have to turn it off for few mins and the use it again...it has beeen like this since xperia z3+/4 and z5 with SD810 under the hood...Sony is very bad at reducing heat in there devices...i compared other devices with same chipset and even if they get warm or hot they still work like they should...why Sony offers us a highend phone with lowend capabilities...i remember the perfomance of my xperia z2...4k recording up to 1 hour no problem ,i could play games more then hour and the phone got warm sometimes even hot to touch but it worked with no LAGS....and last one most anoying is the screen input touch ,just go ahead and compare every phone u can with xperia and ull notice the lag in the screen input...so bad from Sony also XZ is probably my last xperia and i had Z,Z2,Z5 they were good but the competition is far better.and last one Xperia phones should sell for winter gadgets,couse u can warm up ur hands if u need to LOL...cheers guys