Why do people say that bertha lummox runs like lightning?

This comes from the book Lummox by Fannie Hurst which narrates the life of Bertha, an immigrant woman who works as a maid, and struggles with the family she lives with, except for a young poet who is in love with her. this novel explain the dificulties of being someone fighting for a better life. Injustices such as humiliation, sexual assault, and a miserable wage.

She starts going from a rich house to another working as a maid and not lasting in any of them because of different reasons, each time she goes back to the original home she always finds shaming and hate towards her.

So, because of this constant moving, being very unstable, always looking for a place, peace, kindness, is why it is said that she runs like lightning; unpredictable and not lasting when running away.

Let me share a passage from the book which I love and expresses with intensity what the book is about. A very powerful fragment

"The mystery of the will to live. Not a morning but what Bertha knew the terrible reluctance of awakening to reality. She came up out of sleep with a struggle, fighting off another day; turning her back to it and lying there sick with its imminence as the muslin portieres began to lighten. To come out of sleep so enamored of it that to feel the lids lift back from the eyes was torture. To die a little every night and yet fear the beauty of death more than the starkness of life. Sometimes when Bertha came out of sleep, fighting and sobbing against the awakening and grimacing face of Willy hung over her terrible with reality, the impulse to throttle herself back to sleep seemed too strong to withstand. And yet that wink of dawn against the muslin. The tiny vibratory messages of life that were waiting to run across the floor when her bare feet would swing down to it. Ah, that will to live."