No its not a bug, its been like that since touchid was introduced with the iphone 5s.
Once your used to it it takes no longer to unlock than it did previously. You press the homebutton and then release but don't lift your finger off until touchid is registered.
Who knows Apple's actual reasoning, but my guess is that they think the majority of people "wake" an iPhone by pressing the home button, so why not have the iPhone locked all-of-the-time and just use Touch ID to authenticate them every time they "wake" it?
Because when you have touch ID, you don't have to worry about taking time to type in a passcode, and anyone who takes your phone isn't going to be able to get into it.
I used to set my passcode on 15 minutes just because if I locked and then had to use my phone again really quickly, it was a pain to type in my password. With touchID i literally just have my finger on the home button for 2 seconds and I'm in instantly.
The way apple implemented the feature, touchid won't unlock the phone unless the passcode is required. So if it didn't require the passcode inmediately, you'd have to slide to unlock
I love the Touch ID for purchases but I really don't need my screen locked at all.
As it stands now the best combo for me is the " 4 hour option ", but I have to type in passcode. I wish I could use Touch ID with 4 hour option.
Kind of doesn't make sense that i can not.