They probably don't want to use the necessary processor to do it because it costs more so you end up with laggy interfaces instead.

I can't imagine that they're using a powerful enough processor, and their code is just so bad that its slow.

It's clearly doable, Tesla has a snappy smooth interface. It still struggles a bit on pre ryzen cars with the web browser or video players like Netflix, but the actual car interface for car things runs fine and has for a very long time now.