Progress gets lighter after repetition starts carrying part of the load.

In the early stage, every part of the work requires your attention. Later, repetition has automated pieces of it, and those pieces now happen without cost, so the same task takes less out of you. Nothing about the work got easier. You simply stopped having to supply all of it yourself.

If something feels unsustainably heavy, ask how many times you have actually done it. Often the answer is very few. Give it a set number of repetitions before deciding it is too much, since the load usually drops noticeably around the point most people quit.