The amount of time it took to get a letter across the country/world also made a difference to their signal to noise ratio. It especially comes across in the let

The amount of time it took to get a letter across the country/world also made a difference to their signal to noise ratio. It especially comes across in the letters written by thoughtful people. You could compare it to speech making or say a TED talk. The speaker knows she has one shot to make a mark, so a lot more effort goes into it. And it ends up leaving a mark.