Reading the letters of Alexander Hamilton, one finds paragraphs like this:
"The influx of foreigners must, therefore, tend to produce a heterogeneous compound; to change and corrupt the national spirit; to complicate and confound public opinion; to introduce foreign propensities. In the composition of society, the harmony of the ingredients is all important, and whatever tends to a discordant intermixture must have an injurious tendency."
If I'm doing my math right, the average word length here is 5.7 characters, compared to an average of 4.5 - 5 today. Did Americans back then - the rich ones, at least - speak this way too? Or was this kind of language purely written, the way people kept writing Latin long after they stopped speaking it?
Agree with Bodark43 that there were many America subcultures and no recording devices. Trial transcripts provide a rare glimpse as to how people spoke. The Salem Witch Trials transcripts from 1692 suggest the speech had distinct differences from our modern usage. The auxiliary verbs, for example, were different -- using hath instead of has. And they were more apt to use Old English words.
Example: Court: "Hath this woman hurt you speaking to the afflicted...They say you bewitcht your first husband to death."
Bridget Bishop: If it please your worship I know nothing of it."
More transcripts at: http://salem.lib.virginia.edu/texts/transcripts.html
Broad question- there were a lot of Americans , no recording devices. Certainly, some of the most admired writers of English used quite Latinate vocabulary, even when they were being succinct. Like Samuel Johnson: "Almost all absurdity of conduct arises from the imitation of those who we cannot resemble".
But it would be common for someone to dress up their language when formality was expected- and formality in the 18h c. could powerfully motivate almost everyone, not just the rich. Hamilton is writing to impress. On the other hand, here's John Fitch , the unlucky steamboat inventor, writing in an informal narrative way about his captivity by Indians in his memoir, which was never published...
...It seemed as if I was allotted to die by the hand of savages and that I did not care how soon and did not want to gratify them in any one thing altho in the afternoon I was amazing Timorous and would have given all my Lands in Kentucky or nearly all to have saved my life..
The Autobiography of John Fitch, ed. Frank Prager, p76
And here he is writing as formally as he can to Congress in 1787
The subscriber humbly begs leave through you, to represent his situation, and reasons why he has presented a Petition to the United States, praying encouragement to his Steam Boat. I beg leave to inform you, Sir, that my imbarassments is really such, that I am under the necessity of praying Congress to give me some assistance....
Life of John Fitch, Thompson Westcott, p. 199
Fitch is a complex character, but his origins were plebian and he was largely self-educated. You can tell he tried his best to polish his style for the occasion, and there would be a similar distinction between everyday speech and formal speech for most people. Fitch obviously could not write formally very well ( though his attempts were many and often quite long), so it's also possible to see why such people as Fitch, mangling formal English, could be made figures of fun, like Sheridan's Mrs Malaprop.