The 6/30 LineageOS took ~4hrs to do the in place upgrade but ended up bricking my phone.

But this time I'm fully prepared! Side loaded from my dad's PC which I set up last week ago to try out TWRP and reflash Open GApps.

It only took 1hr, mostly from sideloading, and also reloaded a newer Open GApps.

And now my GPS location issue is fixed... Which is the main reason I keep updating...

4 hours?! Did you built it from source on a potato?

@saucyatom no, used the system updater app

@donuts So then I assume the download must have taken so long. Personally I've had issues with the updater (and now also running my own builds), so I ditched it long ago and always flash in twrp. Shouldn't really take much more than 10 minutes - except for the data backup, if you do it.

@saucyatom nope.. have Download and Install steps are sperate. The Downnload is fast, 10mins

check the installation script at zip/META-INF/com/google/android/installer-script, if there're some additional commands, just remove if you don't need them, generally there're only some operations:
check the product code
umount boot, system
write system
write boot
clean up
the most time-consuming step is to unpack the system image into system partition, and the image was encrypted since Lollipop and compressed with brotli since Oreo (it may takes more time to extract). 10min is enough for me to install the system and opengapps... (the script is modified because the package is not for my device but it worked perfectly..)

@juliandroid how's that diff from a clean flash though?

Not really sure how the new? Android in place upgrade process works, letting u upgraded while still using the OS.

@donuts clean-flashing is clean the partition DATA(except internal storage) after flashing the system? And since it took 4 hours, i think the system updater may did some extra opetations, like refreshing Dalvik Cache, which is also a time-consuming operation to convert .dex (except system app which is pre-odexed) into Linux executable format oat, using dex2oat located at /system/bin/dex2oat, caused a long time to update..

@juliandroid clean flashing I mean sideload the zip from TWRP. I don't format data so end result should be the same as the in OS upgrade.

I guess just it flashes to the system, boot partitions directly without needing a temp it leaving a mess behind.

Refreshing dalvik is just deleting it? TWRP does it in a few seconds. Run after the TWRP upgrade was pretty fast.

@donuts yep the same.
and in common flashing dont't need tmp but sideload needs it to store .zip file

Refreshing Dalvik is deleting all the oat and re-creating them.
See, since Kitkat, Android provides a new app running mode called ART(Android Runtime) to optimize battery life and apps, and since lollipop, Android uses ART as the only one mode and removed the old mode called Dalvik(just a mode), so when you see a dialog said "Optimizing Apps", the program dex2oat is running background, and it usually takes a very lone time.