User blog comment:FairyRaven/Fairy Tail Chapter 408: The Definitive Demon, Review and Commentary/@comment-24277053-20141110135803/@comment-7418318-20141111091910

I don't think Erza can be taken to deduce the average case of anything, including this matter.

When Lucy managed to cast Urano Metria without a "knowledge infusion" she had to get help from Gemini and it still wasn't as strong as in the fight against Angel.

When Cana got Fairy Glitter for the first time, she was able to cast the spell, but still couldn't get Blue Note. Now this might have been due to the difference in power, or it might have been due to her not doing everything right. Considering how she was able to out-shine even Jura later on, my money's on the latter.

As you said the Dragon Slayer got one additional element to the magic they knew and were very experienced in, rather than completely new magic. And the stuff they did with their 2nd element seems rather basic. For the most part they just added the 2nd elemental property onto one of their base elemental spells. The only exception would be Gajeel diving into shadows (which also seems one of the stock-abilities of Shadow DSM) and F-Rogue's Rough Silk, but in all likelyhood he had six years to master White DSM before traveling into the past.

Come to think of it, Natsu might even have been bad at using his LFD Mode at first considering how he was even pretty much taking himself out of the battles the first two times he used LFD Roar. With more taining he might have been able to better controll how much magic he put into it and thus not completely use up his remaining magic power with just one spell.

Jellal certainly can't be taken as any kind of example, because he had Ultear sitting in his head.