The old ratio system still works for modern boats. The problem is that most modern boats are made to be fast, sleek looking, and cheap
to build. All this equates to flat undersides with a bolt on keel
, which generally yields a low DLR. Compare that boat with a similar sized boat with a large DLR and you'll find the boat with the higher DLR will generally have a more comfortable motion at sea when there is decent wave action.
DLR is simply a description of how much of a boat of a given LWL is "in" the water
rather than "on" the water
. It's based on the boat's LWL and displacement. A heavier boat which is has the same LWL as a lighter boat will have much more boat below the waterline. This means that it will get thrown around less by the seas and wind
than a lighter boat of the same LWL, which basically floats around like a cork with a comparatively flat underside and will get tipped and turned easily with the wave action as compared to the heavier one with a more rounded and deeper hull
(notice I didn't say keel). This results in a less comfortable ride in moderate+ conditions than a deeper, rounder hull
The heavier displacement doesn't necessarily mean slower either because that comes down to sail area vs displacement. For the same sail area and displacement then yes it would be slower in light winds due to more underwater friction.