Both statements could be true - I wouldn't be too surprised to see Apple stretch the truth; yes, it's true, performance on a 25 second boot (my iphone 6s) from cold was improved to 24 seconds. Doesn't really move the needle, but still true, to some degree. A second here, and a second there - starts to add up though, particularly on boot up, for those of us who end up doing that multiple times a day.
Also, in general, any time you can remove code from a system, that isn't contributing in any meaningful way, is just a good thing to do - both from reducing attack surfaces, as well as general reduction in code size, and the advantages that come along with that.
I was curious so I benchmarked "decrypting" kernelcache.release.n66 on an iPhone 6S and it took about 60 milliseconds to decrypt. It wasn't encrypted in the first place so the decryption results in garbage, but it should be a valid benchmark. The quick boot time with iOS 10 sure is nice, but it isn't because the kernel isn't encrypted.
Also, in general, any time you can remove code from a system, that isn't contributing in any meaningful way, is just a good thing to do - both from reducing attack surfaces, as well as general reduction in code size, and the advantages that come along with that.