Please use this thread for any discussion about the soon-to-be-released island of Altis. I will endeavor to keep this post up to date with any official info that comes out.
When will Altis become available?
Altis is available right now on the dev branch.
How will Altis affect performance compared to Stratis?
As is the case with all larger islands, Altis comes with a framerate hit compared to Stratis.
Your view distance probably plays the biggest role here. Even if you are able to run Stratis with 12k view distance, Altis will probably kill your framerate at this setting due to the significantly higher number of everything.
Reducing your draw distance is the obvious solution.
How big will the download be?
~2.5GB, perhaps more if you're switching from the main build.