I know exactly why it runs slow - I'm on a 2nd gen ipod touch, which will only ever go up to ios 4.2.1, the new DS 1.4 requires ios 4.3 or higher, so I have to stay on 1.3, and it's been getting worse the more habitats and dragons I get. Turning it off and back on again never makes a difference, neither does a hard reset; it's just too old for the latest updates to these games.
Selling a few dragons has helped a little, I was hoping selling a building I don't want to use yet would help a little more. I'm saving up to hopefully get an iphone 4 in a couple months, and was gonna buy it back later, but apparently I'm keeping it whether I want to or not. Thanks for such quick answers!