IIRC tags databases are big files that need to be parsed and operated on via Elisp, so a faster interpreter would be beneficial there. I don't think a faster interpreter would hinder integration of external tools, given often they do not only provide speed, but also quality software with proven longevity.
Traditional ones, yes. However, modern tag databases are just that, databases. Sending a query to a database to find all callers and such can be offloaded to another process easily enough.
Note that this isn't really that new, for the modern tag. Cscope was already an external command. And you already offloaded the tag creation to an external process. I see very little need for the tag navigation to all be in elisp.
The final selection, of course, can still be in elisp. Since that will be user input driven.
