The part $\mathit{A}\mathit{B}$ represents the rise in temperature of the substance from initial temperature to temperature ${\mathit{t}}_1$. The part CD represents the rise in temperature from ${\mathit{t}}_1$ to ${\mathit{t}}_2$. It is clear from the diagram that raise in temperature of part $\mathit{C}\mathit{D}$ is more, so it will take more time. Therefore the slope of $\mathrm{CD}$ is less than slope of $\mathrm{AB}$.