The DRM doesn't make the decode harder. The files used less CPU/GPU power because they weren't as well compressed.
They probably weren't as well compressed because they're intended to work on the older iDevices too which couldn't handle something tuned specifically for the iPad or, even more so, the iPad2.
They probably weren't as well compressed because they're intended to work on the older iDevices too which couldn't handle something tuned specifically for the iPad or, even more so, the iPad2.