Don't waste time for factory reset. Most of the time, it do not work.
When android update, it will clear all the cache, basically format only work if something is corrupted. But when you go check xda and sony forum, many peoples are shooting there, mean it definitely a rom problem rather than individual problem.
And giving you the more reality case. Few pages back, i said my phone was sent to warranty. The phone was lag EVEN first day getting back, which was formatted and upgraded to android 7.1

I don't think it's rom. if it's rom problem, every single one of us will have the same exact problem

Aaaaaaaand this is how hard reality slaps me in the face. Then Sony better fix the ROM asap. Or we can just hope that they will.

After reading tonnes of forums & comments, i guess i found the culprit of the lagging / high ram usage. It caused by the google assistant (OK google). I disabled it and quickly release a lot of ram (2.5 > 2.1). The responsive of switching apps is back.
Will test further for next few days and see how is it.

how do i disable the apps? as i can't find it in the apps setting.

After reading tonnes of forums & comments, i guess i found the culprit of the lagging / high ram usage. It caused by the google assistant (OK google). I disabled it and quickly release a lot of ram (2.5 > 2.1). The responsive of switching apps is back.
Will test further for next few days and see how is it.

It was actually one of the few apps that I already disabled, but the performance in terms of gaming is still sub-par for a phone at this level. I even disabled facebook, zalora, tips & tricks and a few other apps. It really doesn't help much. 

Already turned off the Google apps, phone got smooth only for a few hours.
Can't last for a day.

Now i need to restart my phone for every few hours.

Same. My phone super lag again after 12 hours. Turn off Google app just slower the memory consuming, the main culprit still probably is memory leak.
When the time super lag, i check the ram usage again. It has 0 free ram, and system won't release free ram again.
I restart the phone, and system slowly release the ram. Around 10mb ram per secs, until the usage drop to 1.7gb ram again. Phone is super smooth again, and the ram is slowly go up again.

During the event of super lag.

did you tap into the details and see which app using the most RAM for the pass 3 hours?

Oh yeah guys, any recommendation for tempered glass protection for the XZ/XZs that can be obtained locally?

I passed by Lowyat recently but the 'official' Sony ones look pretty meh to me.

Same. My phone super lag again after 12 hours. Turn off Google app just slower the memory consuming, the main culprit still probably is memory leak.
When the time super lag, i check the ram usage again. It has 0 free ram, and system won't release free ram again.
I restart the phone, and system slowly release the ram. Around 10mb ram per secs, until the usage drop to 1.7gb ram again. Phone is super smooth again, and the ram is slowly go up again.

During the event of super lag.

Oh yeah guys, any recommendation for tempered glass protection for the XZ/XZs that can be obtained locally?

I passed by Lowyat recently but the 'official' Sony ones look pretty meh to me.

Just curious, the tempered glass sold at Sony center how much ya?

i bought mine from Lazada, rm25 if i'm not mistaken. pretty decent for me.
Blue color curved tempered glass.

Around... 80-90+ ringgit :v and it felt flimsy when applied to one of their demo units.
Thanks bro. Complete coverage ke?