newer update is up.

TrackID is closing down. Using since K800i.

Track ID became worse last 2 years.
It was good as the other many years ago.

Same. Especially when the phone become warm, greatly reduced the cpu performance. But seem like only happen after android 7. Android 6 was better performance.

My old z3 is much stable than xz.

I am using it nake after warranty.
Using IMOS as welll

It is getting lag and hotter since 7.0
I can't play game with data now, only wifi. If not after 20mins, the performance dropped a lot until the game lag than my nexus 10, a 4-5y product.
That's why i said it is not stable than my z3.

The first OEM get new android is getting this type of performance -.-

Thinking to downgrade to 6.0, if next update do not fix this.
My z3, top record 50 days without restart. Current need restart every 1-3 days, if not will very lag for multitasking.

New update is up.

Speed like faster after the update. Will look few more days to conclude.

But phone still warm/hot easily than before.

I retrieve my word, phone is starting to lag after almost a day.

Comparing ram is actually useless for android. It is different environment compare to windows. Ram is acting cache for android, for you switching the app faster and battery saving. Most of the time, ram usage is more than 80%. Even my nexus 10, only 1gb ram, it won't lag like my xz.
From my exp, it is mostly ram management issue or memory leak for the rom.

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

You can try if you want to, but please don't put too much of hope. Format phone is the laziness and simplest answer from CS. Just like when you call to ISP, they will ask you restart the modem.
Of course, a clean phone is smooth and faster, because it is empty. And when you install/restore the things, it is doing 1 by 1, step by step. It is lesser burden for the CPU. You feel smooth all the ways. But dirty update, because the phone was wiped the cache. When first run, the first few hours, up to days, it need regenerate the cache in the background. It do all at the same time, so you will feel warm for the first few hours. After the thing is settle down, the phone will not lag and battery-consuming. After the phone fully restored, either clean/dirty install, they are almost the same. But i agree, dirty install has some residues/rubbishs remain there.
That's why i said, unless there is corrupt, or setting error, mostly format won't work.

You can read the comments here from

https://talk.sonymobile.com/t5/Xperia-XZ/XZ...ot/td-p/1227362

For me, i quite confirm it is the rom issue. Some errors may not triggered by everyone. It can be a combination of apps + user patterns + setting to trigger it.
And i said before, my phone was sent to warranty, it was formatted + upgraded latest android that time. It still lag at the first day.

The newer version after 2 days of using, performance is not good as 7.0 and before, but much better than previous version.

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.

Turn off in google app.

My phone performance back to acceptable level after updated to the latest version + disable google assistant + turned off smart cleaner.
play game no longer lag but hotter than before. So far is ok. But disappointed with a flagship just like this performance.