The cemetery, the meadow, that mountainous area on the back of the island, that bay where Mareanie is found... really, they've been mostly confined to a few areas.
Because what's to stop them from rushing through the other islands at a similar rate? Let's say Ash does win against Hala and leaves for Akala in episode 15-20. IMO, with everything Akala has to offer, Ash should not be spending fewer than 40 episodes on there. If they were to rush it, he'd only be on there for around eepisodes. At that rate, we'd be 45 episodes in, and Ash would be done with two islands. Conversely, I don't want to see him spend like 50+ episodes on Akala because what is he going to do on there that's going to do on there that's going to take that long? At least on Melemele you have the school stuff going on and all these other characters to interact with. That will be virtually lost when he gets to Akala.
Either way, there will be a stretch of filler at some point.
Because they're all longer. The same rate would imply, then, that every 9 episodes we clear an island. If not, you're looking at roughly 30 episodes at worst, but I doubt that would happen. Barring DP, we've traditionally cleared the first gym battle at a very quick rate, and the only reason we didn't in DP was because we had Paul and Contests to fill in that time. Clearing the first trial/Gran Trial in 10 episodes is about par for the course and actually feels far longer than usual. There's really nothing to worry about in that regard. Besides, we've no idea how they plan to handle the villainous plot yet, which does factor things...