Yeah, I get the point of her character, I am one of the few that get. But they took so long. They did should have made something like this:
XY014-17 - Camp arc
XY022 - Pancham capture
XY030 - First showcase(lost)
XY035 - second pokémon capture
XY046 - second showcase(lost)
XY060 - Braixen’s evolution
XY066 - third showcase(win)
XY070 - third pokémon capture
XY082 - fourth showcase(lost)
XY085 - pokémon evolution
XY088 - fourth pokémon capture
XY090 - fifth showcase(win)
XY0108 - sixth showcase(win)
That more or less ignores her character trait. XY 1, 2 and 7 show that Serena has a problem with committing to something and getting motivated. To have her therefore make a decision just 14 episodes in, a mere 7 episodes after she joins removes that entire trait. It more or less proves what I said about people more wanting Serena to develop the way they wanted her to, rather than the way she did, particularly since her conflict is largely internal. Besides, it's kind of obvious they'd have wanted more Showcases but got blindsided and had to make sure everything was done in that last year while still being a marketing show.
Actually the depression arc was Dawn’s strongest development. In contrast to May and Serena the writers had no problem allowing people to outclass her constantly. May appearing was the finale of Dawn’s Depression arc which allowed her to move on strive from there on out.
Perhaps I misworded: if DP had ended at episode 40 Dawn wouldn't be developed well because being in the middle of the depression arc has no payoff until May. It's hard to say she developed from that, or rather that her development had meaning, until the Wallace Cup. Not that the depression arc wasn't important (because it was), but that just stopping in the middle of it makes her, relatively, equally as developed as Serena at that point, and that a Serena 100 episodes in would have more development than a Dawn in the middle of her depression. Apples to apples, it's still a hard judgment call since Dawn still had 100 episodes left, largely developed as a trainer/Coordinator than a character after the Wallace Cup, and was done with the end of the Grand Festival, while Serena got focus through the Master Class, developing her character more, and that development lasted through to the final episode (and yes, it did).
Both are good, but I don't think we can compare them on a "how many episodes" basis. It's the same reason we can't fully compare Ash's quest when his first gym battles in XY and BW were in their 6th episodes, while DP was the 18th. It's just not a like comparison.