I wonder if some of that impression isn't anachronistic projection, though.
Today you think of a letter as something that takes time/energy/thought investment. But it was not at all uncommon for frequent letter-writers of previous eras to write many letters in a day, closer to the way we write emails than the way we write letters. Probably not
quite as routine as the average email, but people who wrote thousands of letters during their life treated it as just as normal way of communicating. It wasn't that strange for letters to be dashed off in short periods of time, and like with email, the amount of agonizing/revision/thought that was put into a particular letter varied greatly.
On your latter worry, it seems like there's some of that with earlier eras' figures as well. For example, some people's opinion of Simone de Beauvoir and Jean-Paul Sartre has shifted for the worse since their correspondence was published, since much of it is more like a sordid chatroom log, full of offhanded gossip about other people and such, than like the philosophical works they intended for public consumption.