From "Available on this device" to "Always available on this device"

which of these results in the fastest access when you are online with internet access and logged in to one drive?   I am having very very slow response to getting files.   I guess asked in a different way what is the order of access ?  i.e.  if file is local (always available on this device) does the file get sourced directly from your hard drive?  then if it changes is the sync to the cloud(onedrive) done asynchronously in the background  both during access/changing,  and at time of save.    is the save done to hard drive first then synced in background to one drive,   or saved to one drive then synced to hard drive.     the reason I ask is that depending on the speed and reliablity of your internet connect it can make a difference.   I cannot find any info on how the internals seem to work.    since i converted I have noticed significantly slower access ( could be something else, but need to understand the details here).          

Salvatore Biscari 
this seems misleading. At least in the way I understood you. I mean the statement that "system will never automatically delete the local copy" for "Always available on this device" option.
I have MS Team and there are some files. I synced sharepoint folder containing those files to my laptop. I chose "always available" option in OneDrive. Then I created a copy of a 1 file on my machine, this file appeared in MS Teams. Then I deleted the file using MS Teams. I expected that this file will remain on my local machine as you suggested (system shouldn't delete local copy as I understood). But it was gone...
Maybe I misunderstood something? But it doesn't look like always working this way (at least with syncing of Sharepoint folders)

UPD: after some test I found how it works. So if I created some file using online version of OneDrive - it will be automatically downloaded on my machine to the folder which was marked as "always available". But it will not be automatically downloaded to machine if folder was not marked with this option (always available). So it works one direction but doesn't work another. Hope it's clear

This time (end of 2017) there is no difference between the two diffrent status (local available) and always available). The difference is only a different Attribute in NTFS.

I have written 2 articles in German language, (have had no time to translate it to english)

The 3 Status of OneDrive Files on Demand (Part1)

2nd part is going a little bit deeper into NTFS

so try it with google translation

Hans, MVP Office Servers and Services, Mr.OneDrive

Thanks Salvatore. It does make sense but still raises some questions. What does "when needed" exactly mean? And where is this documented?