It's up to the CPU actually and most CPU's have the AES instructionset by default.

In this case, the performance hit is minuscule.

It does slow things down technically, but not by much. You won't notice it.

OS X and iOS heavily leverage the AES-NI instruction set (and whatever the equivalent is on ARM), so performance degradation is minimal.

It's noticeable for sure. My Samsung 830 SSD took a pretty serious hit in transfer rates. New full flash based Mac's like my late '13 retina i7 seems to handle it pretty well, though, there's still a noticeable difference. Worth it? Absolutely. If you do end up doing it - when prompted, I would recommend against saving your encryption key to iCloud.

I don't feel/see any performance loss at all. (I'm developing audio processing software).

I guess there is some performance degradation but unless you constantly max out your SSD you probably won't notice anything.

Just turned it on on my 15" 2012 rMBP and it finished encrypting the drive. Before I was getting around 520-540MB/s read/write. Now it's 440-460MB/s. It's not a large hit, slightly noticeable, but worth the security. I am not sure about battery life, but I could not see any one else online complaining.