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We investigate how a combination of a nonmagnetic-impurity scattering rate$\gamma$ and finite subgap states parametrized by Dynes $\Gamma$ affectsvarious physical quantities relevant to to superconducting devices: kineticinductance $L_k$, complex conductivity $\sigma$, surface resistance $R_s$,quality factor $Q$, and depairing current density $J_d$. All the calculationsare based on the Eilenberger formalism of the BCS theory. We assume the devicematerials are extreme type-II $s$-wave superconductors. It is well known thatthe optimum impurity concentration ($\gamma/\Delta_0 \sim 1$) minimizes $R_s$.Here, $\Delta_0$ is the pair potential for the idealized ($\Gamma\to 0$)superconductor for the temperature $T\to 0$. We find the optimum $\Gamma$ canalso reduce $R_s$ by one order of magnitude for a clean superconductor($\gamma/\Delta_0 < 1$) and a few tens $\%$ for a dirty superconductor($\gamma/\Delta_0 > 1$). Also, we find a nearly-ideal ($\Gamma/\Delta_0 \ll 1$)clean-limit superconductor exhibits a frequency-independent $R_s$ for a broadrange of frequency $\omega$, which can significantly improve $Q$ of a verycompact cavity with a few tens of GHz frequency. As $\Gamma$ or $\gamma$increases, the plateau disappears, and $R_s$ obeys the $\omega^2$ dependence.The subgap-state-induced residual surface resistance $R_{\rm res}$ is alsostudied, which can be detected by an SRF-grade high-$Q$ 3D resonator. Wecalculate $L_k(\gamma, \Gamma,T)$ and $J_d(\gamma, \Gamma,T)$, which aremonotonic increasing and decreasing functions of $(\gamma, \Gamma,T)$,respectively. Measurements of $(\gamma, \Gamma)$ of device materials can givehelpful information on engineering $(\gamma, \Gamma)$ via materials processing,by which it would be possible to improve $Q$, engineer $L_k$, and ameliorate$J_d$.

Effects of nonmagnetic impurities and subgap states on the kinetic inductance, complex conductivity, quality factor and depairing current density