The quantum speed limit time τ_QSL (black-solid line) and the excited state population p(τ) (red-dashed line) as functions of the initial time parameter τ for the initial excited state. (a) Δ = 0.4; (b) Δ = 0.9; (c) Δ = 1.1; (d) Δ = 2.5. The actual driving time τ_D = 1.