1. Yes, touch screen feature is supported in the current spacedesk version.
2. Yes, spacedesk supports multiple viewer connections (except when connecting via spacedesk HTML5 Viewer).
3. Not exactly the app, one of the main factor that affects the performance is the network connectivity. If spacedesk is connected over a slow wireless network, then it will result to slow performance.
The ideal spacedesk connection should be over a wired network, please check the performance tuning chapter in the user manual for various suggestions on how to improve performance https://www.spacedesk.net/user-manual/#item_505.
FYI: Native Android USB cable support will be available soon.