A Japanese proverb, "石の上にも三年" (Ishi no ue ni mo san nen), translated to English, is the title of this article. It carries the general meaning of staying consistent (working towards a positive direction), staying the course through difficult times, eventually, things will change.
Perhaps in the Iaido context, it may be useful to think of this perspective: "that the longer the timeframe, the less one needs intensity and more one needs in consistency". A minimum level of intensity or rigour in practice is needed, but being consistent is foundational. It is doing it (practice) when you don't feel like doing it.
Marked improvements don't happen overnight; over a time horizon, improvements may be viewed as an (advantageous) divergence from a norm. If one wants improvements (advantageous divergence), one has to study the basics (Kihon, Kata) and continue to practice them on one's best and worst days.
Consistency brings about a deeper level of self-awareness, and that in turn leads to refinement in overall physique, mental & spiritual training and understanding of technique.