Downgrading an A7 device (like the 5s) below iOS 10.2 typically requires disabling the SEP (Secure Enclave Processor) , which handles security-sensitive data. This causes:
The iPhone 5s has a non-replaceable battery. If you are downgrading a used device, the battery health is likely degraded. While iOS 7 is lighter than iOS 12, a dying battery will still result in poor performance.

