Correct me if I'm wrong. I watched the Safari Content Blocker video that is presented in WWDC 2015 and it mentioned that the list of content to be filtered is compiled to bit code instead of reading it as a JSON file, which makes it more efficient and less draining on CPU. Since it is compiled down to bit code, 32-bit will not be compatible to 64-bit and that's why only the newer iPhones and iPads are compatible. It is not that iPhone 5 is not powerful enough but simply the CPU architecture doesn't support.