A show can be 23 episodes long if they just drop over-arching storylines and do short arcs instead, like in a comic book. From what I hear Agent of Shield is doing this.
If you reduce a Superhero show like the Flash down to 10 episodes a season, then it wouldn't be much of a superhero show. He has a rogues gallery, you want to see him fight those guys on the screen week to week, that's why it's a superhero show in the first place. It's just a matter of doing it well, give the villains proper development instead of the same old concept. One awful thing the show does is that they bring in new villains who has NOTHING to do with the Flash (King Shark, Rupture, Atom Smasher, Killer Frost, Hannibal Bates etc...), instead of wasting an episode on those guys, why not just use Flash's regular rogues and give them good character development instead and go from there.
For a show with crazy time traveling and dimensional hopping concepts, they still play it way too safe.