Tip: On some older, fill-rate limited devices such as iPhone 4 and the original iPad, you might want to go to Jewels 2 Settings -> Advanced Settings and tick the reduced resolution box, then exit and restart the game. Also turning of the "Water Effects" helps if the game feels sluggish.
Another tip: If you happen to have several devices such as a phone and a tablet, do check out the awesome game data sync feature in Jewels 2, and have your game profiles and saved games synchronized across multiple devices! Just sign in with Google+ (from the main menu) and you're good to go — it's especially cool since it works across platforms: start a game on your Android-phone and resume it on your iPad, for example. 🙂