The statistics reported by GitLab for the x264 repo (https://code.videolan.org/videolan/x264) report that the project is 13.5% assembly; common utilities used in the inner loops of the codec have optimized assembly implementations for several CPU architectures.
A lot of the encoding side on ffmpeg now uses hand-coded assembly optimizations to take advantage of avx512 instructions on newer x64 processors for "100x speed increase" since February 2025 in a stable form
