In life course studies five principles guide social science researchers: (1) the principle of human development and aging, (2) the principle of human agency, (3) the principle of historical time and space, (4) the principle of timing, and (5) the principle of linked lives. We propose a sixth principle: life course tempo explicitly depends on other life course principles especially the external principles of (2), (3), and (5). Tempo changes may have sociological and psychological consequences. To demonstrate the sixth principle at work, we analyze a sample of the peasant letters both to and from America in Thomas and Znaniecki's 'The Polish Peasant in Europe and America', the pioneer life history study of Polish immigrants in early 20th century. Two types of tempo change in transition into first marriage are evident in the letters, waiting/postponement and haste, which resulted from changed historical time and space and reorganized human agency of the immigrants. Thus, this research is inspired by Thomas and Znaniecki's work on the Polish peasant and Znaniecki's methodology and in turn uses the Polish peasant letters as data.