I have Photogene on my iPad mini 1st generation with iOS 8.1. I have no issue with speed, I found it decent for a "2 years old device".
I use first the Apple photo app to import the pictures to the iPad. On the photo app you only see the JPEG pictures but it import both JPEG and RAW. Make sure you have enough space on your iPad.
From my experience the original pictures are not deleted or removed from the SD card.
I then use the Photogene app to edit the pictures. It seems that Photogene open the RAW, because it says "decoding RAW...". Photogene doesn't alter the original raw file, it create a "layer" with the adjustments you have made, you can comeback to the original image anytime you want. You can then export the image in many different way.
Once connect to my Mac. I use the iPhoto transfer the photo I have adjusted and exported. BUT iPhoto doesn't transfer the RAW to my Mac because it cannot read it.
In general I found not to be limited by the speed but by the IPad memory available as the Apple photo app doesn't let you the option to import the JPEG only. I'll be happy to only copy the JPEG especially when I'm traveling where I just need to visualize what I have shot.
In addition, copying JPEG + RAW can be quite long when you have a full 32GB SD card.
In my opinion The iPad is a very nice tool to review your photos on trip without the need of a computer.
I have to try other IOS photo app like Photoshop.
I'll be glad to hear more form others.