Guys, just to update, everything is fine now. Enjoying Android 7.0 on my Zenfone 3 5.2".
The upgrade file is around 1.5GB I think.

Okay, what people do need to know is that after updating, the wifi settings will be altered without it being mentioned. Previously installed Unifi in my home, but somehow certain gadget will have trouble using the wifi. This is because the Default gateway couldn't automatically sync between router and the gadget. What was needed to solve the issue was to manually set the default gateway and IP for the gadget.

My Zenfone 3 was manually set to use the wifi when the Unifi was first installed, which was a few months ago. So, after update from Android 6.1 to 7.0 was completed on my Zenfone 3, I'm still connected to my wifi (which means that network settings wasn't totally reset), but I can't connect to the internet.

Since I didn't remember that I need to manually set back the Default Gateway and IP, I first try to Completely Reset my Network Settings, which makes the phone forgets all the saved network. This is to no avail, which I then continue with do a full factory reset, which also is to no avail. Then I proceed to hard factory reset and clear cache by holding down power button and volume down button for 6 seconds when phone is off. This also didn't result in my phone can be connected to internet using wifi. Then, after losing all hope, I remembered to check my TV network settings and try using the similar settings to my phone, and voila, there is internet connection with wifi.

**Important**
So, for people updating their phones to Android 7.0 (for Asus Zenfone 3 MY), do note that it will change the manually set wifi settings to auto.

Well, I didn't feel it improved, though it definitely isn't getting worse. I do feel like the phone is wee-bit more responsive, even compared to when I bought it new. Maybe the animations/transitions has speed up.

It is not compulsory to do factory reset though, but it's probably highly recommended.