Bundles will execute some kind of script. Which will create a new process. Autocompletion syntax highlighting and similar features need to be very, very fast. I don't want to wait a whole second for for an autocompletion list to show up.
I think delegating syntax highlighting up to simple bundle commands seems a bit far-fetched. I don't know if it's even possible to update text with new colors.
As for auto-completions: For me, getting php function suggestions (alt-esc) works absolutely instantly, while it uses a plain Ruby script. Do you notice any lag?
Given the instantaneous results I get, I don't think starting the ruby interpreter can be the source of slow-downs. Without knowing any more, I'd put the blame at first on a slow hard drive. I have an SSD.