This question relies on a lot of bizarre assumptions:

1. That there is a "beginning" to British and American histories so that we can compare the length of time of events from these "starts."
2. That the histories of the United Kingdom and the United States are not a shared history, i.e., that they existed "in a vacuum," as they say. The histories of the English and American people were always intertwined, and indeed many of the ideas and values that made up American identity derived directly from the English. Moreover, both the English and the Americans existed within the context of Western Civilization as a whole. England and the United States have their individual histories to be sure, but they undoubtedly share a history as well. The Magna Carta, for example, was written well before the discovery of the Americas and yet played a role in the development of the Constitution of the United States. This English development contributed to American culture; American culture did not appear out of nowhere but, at least initially, derived from the various cultures that contributed to it.
3. That events in separate nations' histories can be made entirely analogies, i.e., that we can ask "when did the English have their civil war?" or "When did the Americans have their third assassination of a head of government?" as if these were inevitable events that will occur in each society, with the only question being how long it takes until it happens.
4. That points 1-3 can culminate in a question such as "the UK abolished slavery in its empire in 1833, the Americans in their nation in 1865--what took the English so long???" This is beyond absurd. The United States did not exist in a vacuum, and its belated condemnation of slavery must be understood within the context of both global and national trends and the interrelation between the two. In a sense, the English condemnation anticipated ours, and it indeed played a direct role in the evolution of slavery in our country (by ending the slave trade and forcing slavery to go entirely domestic).
5. That something called "societal change," in addition to being analogous, can have a known start. Again, I point back to the Magna Carta -- was this not the manifestation of societal changes? How do you decide when a nation has had a "societal change"? All nations everywhere are always experiencing societal change. Some of these changes happen internationally, as the result of larger social or technological changes. Going back to points I made above, women in the UK and the US gained the right to vote at almost exactly the same time. This was part of our larger shared history.
6. That we can compare the state of English society in the year 1500 to the state of American society in the year 2000 to reach meaningful conclusions. With your math, this is basically what you are doing, saying that if the United States accomplished something less than five hundred years after the English that it means they accomplished it "first." Dude, come on, the English outlawed slavery thirty years earlier and embraced universal suffrage at the same time, but we can claim to have done it "faster" because we are saying they had a 500-year head start? That's just silly.

The United States has England to thank for many of the values and principles we still hold dear today. We share a history. We did not come up with the concept of universal suffrage out of thin air, for example, and therefore cannot say "we did it faster!" Nevertheless, we do have our own national history that has taken us down paths that the English--or any other nation--ever followed. And there is the more interesting (or at least sensible) question, I think: what did we do different from the rest of the world? There's a lot more there than "how did we do it all faster?"