Reduced speed with degraded battery doesn't only happen for phones unfortunately. My Lenovo laptop is painfully slow when unplugged. I thought something is broken, but Lenovo told me it's because the battery is degraded. The consequences are similar as with phones: The battery can't be easily changed and the costs for letting it be changed by Lenovo are not worth it considering the age of the laptop. So I'll likely live with it until I'm annoyed enough to buy a new one.