According to our current understanding of the universe, there is no concept of the expansion of the universe reaching an "optimal speed" in the traditional sense. The expansion of the universe is described by the theory of cosmic inflation and the subsequent expansion driven by dark energy.
During the early moments of the universe, a period of rapid expansion known as cosmic inflation is thought to have occurred. This inflationary period is believed to have driven the universe to expand exponentially in a very brief period of time. However, the concept of speed in this context is not defined in the same way as the speed of an object moving through space.
After cosmic inflation, the expansion of the universe continued but at a much slower rate. The rate of expansion is determined by the density of matter and energy in the universe, including the presence of dark matter and dark energy. The current observational evidence suggests that the expansion of the universe is accelerating, driven by the repulsive effects of dark energy.
The rate of this expansion is described by the Hubble parameter, which quantifies the rate at which objects in the universe move away from each other. The value of the Hubble parameter can change over time depending on the amount and nature of the matter and energy in the universe. However, it is not a traditional speed that can be compared to the speed of light or other objects.
It's important to note that our understanding of the expansion of the universe is based on current observations and theories. Future discoveries and advances in our understanding may provide new insights that could potentially refine or change our understanding of the expansion and its behavior.