If you are using the latest version of the app, the "Decoder" tab might be inside the "Player" section of the settings menu depending on the UI update.
Summary The Mx Player ARMv8 NEON codec provides a pragmatic, performance-focused path to higher-quality, more efficient video playback on modern 64‑bit ARM devices by exploiting NEON SIMD acceleration and ARMv8 architectural features. Mx Player Armv8 Neon Codec