No, I often run multiple uibuilder instances on a single old Pi2 or Pi3 with lots of other things running. Performance will only start to suffer if you run too much on the Pi, run the Linux desktop on the Pi, have massive, complex flows (though some of mine are quite large) or try to send too much data too quickly.
Not, it will be fine as long as you aren't trying to update the interfaces on a millisecond basis. Typically I only update interfaces once a minute for home automation sensor outputs. You can do a lot more than that if you like but it rarely seems sensible in a home environment. Switches and similar controls happen ad-hoc of course without an issue.
One other thing to note. It is perfectly possible to build responsive interfaces that adapt nicely to different sized displays. The default libraries (VueJS and bootstrap-vue) that come with uibuilder make it pretty easy to build adaptive UI's so you might not actually need two different UI's.
Very, very unlikely you would need this complexity, especially with a 4GB Pi 4. You would have to be really loading up the Pi with tons of other services. Even my 512MB Pi2 runs some complex flows, Dashboard and several uibuilder instances along with MQTT, InfluxDB, Telegraf and Grafana. My 1GB Pi3 runs a similar stack and runs the Java-based Ubiquity Unifi Wi-Fi controller as well & that is a beast.
Yes, but the default libraries make this very simple. Very little, if any, additional code is required since the frameworks themselves are already responsive and adaptive and have sensible defaults.
That is true in that you need to deal with the incoming msg's in your front-end index.js file. The examples show how and the uibuilder library takes care of much of the complexity for you.
