The ripple effect of 1955 takes weeks in the year 1955. This is because in those weeks there is still a finite probability that the mess can get corrected. (and it does). The photo and Martin that are disappearing are in 1955. Had there been a way to fast forward to 1968, there would be no slow fading ripple effect in 1968
In the same way, the doc sending out a letter is in the year 1885 and any ripple effect would be felt then in 1885.
Doc planning to send that courier is not shown in the movie hence we don't know how long the slow ripple effect may have been. The courier is received in 1955 is basically equal to fast forwarding to the point of impact. There would be no slow ripple effect at that moment in 1955, it is instant.