Processing map metadata updates to reduce client I/O variability and device time to ready (TTR)

Method and apparatus for enhancing performance of a storage device, such as a solid-state drive (SSD). A non-volatile memory (NVM) stores user data from a client device. Map metadata in a local memory describes locations of the user data in the NVM. The map metadata is arranged as a snapshot and accumulated journal updates. A metadata manager circuit combines a first portion of the journal updates with the existing snapshot to generate a new snapshot, and places a second portion of the journal updates into a read-only journal table in accordance with a selected metadata journaling strategy. A controller uses the updated snapshot and the table to service subsequently received client commands. Only dirty entries are processed and are written at the slowest acceptable rate, thereby improving client I/O performance during normal operation and time to ready (TTR) performance of the device during initialization.