When the time complexity of a computation such as adding two $\lg n$-bit numbers $x$ and $y$ is considered, it is often assumed that the bits in $x$ and $y$ are available all at once unless the algorithm in question is bit-serial and bits of $x$ and $y$ arrive over time. So, while it is true that every bit counts and we can't ignore any given bit, one doesn't need to spend $O(\lg n)$ time to wait for bits of $x$ and $y$ to arrive before the computation begins or in-between the successive bit additions. With this convention in place, $x$ and $y$ can be added using a Brent-Kung prefix adder in $O(\lg\lg n)$ time complexity using constant fan-in and constant-fan-out gates. Brent-Kung uses a particular prefix gate with two inputs and two outputs and $O(1)$ gate delay to achieve this time complexity.
One other point is that time and space complexities of an algorithm or computation cannot not be expressed in terms of implementation specific figures such as address or data bus widths, RAM or register size (number of bits, bytes, words), clock rates, etc of a particular physical system made out of a constant number of components. Such complexities involve variables such as $n,\lg n,$ etc, whereas an implementation of an algorithm on a specific piece of hardware or system can be determined down to a nanosecond if all system and component time and space values are accurately predictable. As far as time and space complexities are concerned, all physical systems built out of a constant number of components have $O(1)$ time and $O(1)$ space complexity.
The classic references on this sort of question are:
(1) Winograd, Shmuel. "On the time required to perform addition." Journal of the ACM (JACM) 12.2 (1965): 277-285.
(2) Brent, Richard P., and Hsiang T. Kung. "A regular layout for parallel adders." IEEE transactions on Computers 3 (1982): 260-264.
The $O(\lg \lg n)$ time complexity of B-K prefix adder is consistent with Winograd's lower bound given in .