The mixing rate (Rϕ) is the temporal rate of increase in the solvent strength in gradient LC. The optimal Rϕ (Rϕ,Opt) for a gradient analysis is the one at which a required separation capacity and peak capacity of the analysis are obtained in the shortest time. The Rϕ,Opt of LSS (linear solvent strength) gradient LC is found in dimensionless form (rϕ,Opt) expressing Rϕ,Opt in units of hold-up time (t0) and characteristic strength-constant (Φchar). Previously unknown effect of the gradient band compression on the peak capacity is taken into account. The rϕ,Opt depends on the solvent composition range covered by the mixing ramp and on the available pressure. A default rϕ at which the analysis time is contained within 30% margin of its minimum at rϕ,Opt for a broad range of conditions is proposed. As an example, the recommended default for small-molecule samples is 5% increase in the solvent strength per each t0-long increment in time. At this rate, approximately 0.2√N units of peak capacity are generated per each 10% solvent strength increment. The effect of a column kinetic optimization is also evaluated.