In fact, this is an exaggeration.
The correct understanding is that after pregnancy and childbirth, pregnant women and parturients consume a lot of physical strength and energy, and things need to be considered more than before pregnancy. Plus postpartum care for children, feeding night milk and other reasons, there will be insufficient rest, long-term exhaustion, so when doing other things, it appears to slow down and reduce efficiency.
Then why is it silly for three years? Simply put, as the children grow up, it is relatively easy to take care of them and their energy gradually recovers.