The workshop file download/update process is handled by steam API. The game ask for file and the Steam provides the latest version of the file. If the file is not changed, the steam should provide the file from it's own cache but I'm not sure what rules apply to the steam cache and how it's handled by steam API.

What I'm trying to say is that unfortunately in current version, the game doesn't have control over the downloading and updating process.

That's a shame, thanks for taking the time to explain!

btw let's say
1)i m playing a campaign DL/ed by Workshop.
2)I have spend lots of hours in progress
3)The developer of the mission uploads an updated version that isn't compatible with the previous version savegames

so..
The update will happen' automatically the next time i ll try to continue my campaign?

That is handled differently. The mission PBO is now part of the workshop mission save. When you load the game, the mission pbo is loaded from the save. If you restart (revert) the mission, new mission is downloaded from the steam.

This way you can resume the previously saved game and finish it without worrying the author will break your saves by updating the mission. If you want to play last version you need to restart the game. In general, there is no safe way to load old save on new mission, because it depends on the changes done in the newer version (new scripts, new units in the map...).

What I was trying to say by my first post is that the updating is now handled by Steam. It could be possible to build another in-game cache, that would keep the missions regardless of the Steam behavior. Also in offline mode, you probably won't be able to download list of subscribed mission, so that would have to be handle differently as well.

Has this been left stagnant? Please look into it again, it is a real pain in the arse having them all load every single time.

Related to: http://feedback.arma3.com/view.php?id=13710

Could the game cache the scenario files itself in the Missions directory (which would also allow for offline playing), and have the download/update not be an automatic process? Perhaps you could update by clicking a button on the scenarios menu in-game. Another option is grabbing the list quickly, but only updating the file at the last minute when you click "Play".

As it is, much to my dissatisfaction it takes just over 30 seconds on my system to update 52 missions. This is way too long, particularly if I try to load a mission that doesn't work and I'm booted back to the scenario list page and have to wait all over. Also missions are always added to the end of the list so I have to wait until the very end to play recently downloaded missions.

I know 30 seconds doesn't seem like a long time, but it's unnecessary friction that mars the otherwise nice experience of downloading or switching to play different missions. Thanks for understanding!

As more and more missions are made, the worse this problem will become. I hope they try to implement Superfluid's suggestion.
The readme.txt file in the Missions directory actually says:
"... You can also place downloaded custom scenarios in this folder and play them from the Scenarios list."
Now we just need a button to give us the option to download custom scenarios.

Well, looks like it's fixed in the latest devbranch. All missions are now cached and updated in background.

O, I see scenarios that I loaded once, will now work when I go offline.
Thank you George_. Since I take my PC home over the weekends were I do not have internet. My weekends promises to be a lot more fun.