It’s simply a mode that is as tuned as possible, it’s also VERY power hungry so it’s not recommended for regular use.
This is an android thing to begin with, before you start complaining how htc have changed things.
Edit: it’s visible if you just have updated your One, but haven’t done an factory restore. Or least it was at mine, but after a factory restore it was not visible.
This way work for all htc phones having 4.2 or above, from One X to One with 4.4.2
Under Settings are now hidden, you enable it by going to Settings > About > Software information > More.. where you touch Build number 7 times. Which will give you a count down, and then say “you’re now a developer”. And Developer Options are back!
Ok, that’s all good. But, how do I disable it again? To hide it again.
Of course I’ve covered that as well. Read more HERE