I expect that would be hard to do because of CONCAT_RES. We'd have to know what concatenations we're using (there are several, depends on the pages) and that'd be a hard list to maintain. We'd inevitably forget one.

Doing the compression on the fly is fine since we cache the results in varnish. We wouldn't be compressing them very often, just when they change or when they fall out of the cache.

Since doing it at build time doesn't save us much, I'd be disinclined to take the effort of doing that.