Hearthstone on Phones

Blizzard launched a Hearthstone version for iOS and Android smart phones a bit over a week ago. The game is as fun on phone as well as on tablet or PC. Blizzard managed to make the game fit onto a small screen with very easy-to-use and smart features. 

I'm quite certain that this will only result in more and more players for Hearthstone as now people who do not have a tablet can use it on their smart phone even when they're not at home.

It seems to work perfectly on my iPhone 6 which is pretty obvious as it is a brand new phone. Hearthstone is quite a large game for a mobile app so I would guess that most users with older smart phones experience some kind of slowness. Anyone tried the new Hearthstone app on an older smart phone?