Reading many play scripts set in the period, it seems that conversations were more slow paced and complex. People frequently misuse words, but due to the complexity of the language these instances are glossed over and not mentioned.
So I wondered whether people merely knew the rough meaning of words, skipped over these as a matter of politeness or if people were perfectly (or no less) fluent (than today).
What do you mean people misused words? Word meaning in English has changed quite a bit between the eighteenth century and now. Are you sure you're not just unaware of the meanings in the time period?
Second, you may want to go over to /r/linguistics for this question, as it's a question of historical linguistics, not of history per se.