Why not scan it before it installs to see if its even safe to install in the first place?
Well that is kind of impossible as if it is not installed yet then it can’t be scanned. If you mean while the app is being installed I guess that might be possible.
I mean like have the app download, then scan it and if safe then install.
Android triggers us only after installation. That is why we scan the application as soon as it is installed. We are working on some other interesting methods to go one step further 
Lets wait and see…
As soon as an APK is DL’d, it wants to install. Scanning at that time would slow the phone down, would it not?
I always prefer to install, even multiple updates, one at a time. But, wouldn’t that affect a batch update task?