When I first picked up the Knights vs. Dragons duel deck, it performed very well. One of my sons has the Elves deck (from the Elves vs. Goblins duel deck). He tweaked it over the past few months to rapidly speed his card deployment. Within about five turns, he had me far outnumbered, even though I was able to get six Goblin tokens out in the first three turns.

I'm looking for ways to improve the Dragons deck, to speed up my deployment of some of its power cards. Aside from Seething Song, what other cards might I include to improve the deck's playability.

Don't get me wrong. It is still a great deck. When the cards fall just right, I can still stop most opponents, but it often (perhaps more often than might be expected) leaves me wanting.

What have you used, or what might you suggest?

Acceleration might not be what you need. As an old red player i learned there's no way to out-ramp elves.
So try board control. Volcanic Fallout and Slagstorm might be your best choices. The creatures in your deck that you care about are mostly out of burn range (you don't care about the tokens and little goblins for anything else than board control)

Ohh and you might need a lighting bolt or two for the lords that will get the other pesty little elves out of volcanic range.