I've noticed, (but have not studied), some loose relationships in
transmission line characteristics (and I guess waveguides fit in here).
From an observer's point of view, it seems that a high characteristic
impedence line (like 400-ohm or 600-ohm ladder line) also is usually a
lower-loss line, and has a higher velocity factor.
It also seems that some coax may have a low VF and high loss.

Is there a real cause for the relationship of these 3 characteristics of
transmission lines ? Is it something we can generalize ?
It makes some sense to say that the faster a signal gets through the line,
the less loss it will have - and that gives some credence to the
relationship in VF and loss being inversely associated.