Trying to remember where this story is from. A girl, concerned about humanity overpopulating the planet and/or destroying itself, wishes for something or someone to make a change. Some kind of alien or inter-dimensional being hears her and does something that affects the world and humanity.
Time dilation occurs and people start speeding up or slowing down. It affects people differently, but those that speed up get so fast they disappear and can occasionally be heard as a background hum and those that slow down become nearly statue-like. Eventually society is reorganized where those that are very slow are moved to their own communities where they can be kept safe, and those that are super-fast have essentially become so advanced they have moved beyond Earth.
Thanks for the help!