I updated to 8 and my iPad 2 lags. The last time I remember that everything was snappy on this device was when using iOS 6.x. Since I updated to 7 it lags and it is noticeable. Updating to 8 did not cure the lag but I think I am going to live with it as I want the new features.
The iPad 2 is a device from 2011 so it is not surprising that it lags, but if you were using iOS 7 your are already accustomed to lag anyway.
I still am not sure if the lag is worst on 8, but I think it is. It is specially noticeable when you change the orientation of the iPad like lets say from portrait to landscape when you are in the iOS main page with the icons. You can first see some animation and then after some lag you see the screen changing orientation. Activating the zoom with 3 fingers double tap also lags. I can not remember this lags on iOS 7.
I recommend everyone to turn off animations and other visual things in settings to make the experience better. I had to do it after updating to iOS 7 a year ago, and if you can, try to update via iTunes and not OTA.
The first hours after the update the device is going to be really sluggish so let it be in peace. It was better after letting it pass the night. Some reset of settings like network settings have helped others with sluggish WiFi. I have still not tried a full reset wiping the iPad with reinstallation of all the apps from scratch, but I don't have time for this. I have hundreds of apps.
Did you also have lag on iOS 7.x? Could you compare the lag on 7 vs 8?
Nice for you. Do you remember how your iPad was with iOS 6? Was 6 better than 7?
I know the feeling