The main objective of UTCDateAndTime is to make DateAndTime conceptually simpler, but a happy side effect is that it is also significantly faster than the old implementation.
Hi Dave, when you first introduced UTCDateAndTime, it was originally significantly faster than legacy Chronology because legacy had a gross inefficiency in it. You work exposed that inefficiency, which was then fixed, pretty much neutralizing the performance advantage of UTCDateAndTime, of that time.
If you have made further improvements since then, I would be interested to see your benchmarks again.
One thing I wondered was, due to greater SmallInteger range, whether your implementation would be significantly faster in 64-bit image. In 32-bit though, IIRC, it no longer is..