For a pretty small N I've found that less clever can be quite a bit faster. I'd try a linear search - possibly SIMD if you can change the data format to struct-of-arrays. An adaptive approach that uses linear search up to a certain N can also yield some benefit.

I know protobuf code is extremely high quality, but I really can't stand the c-style naming conventions.

I know people train themselves into grokking this and reading and emitting this way, but it sounds like writing "bork bork bork bork" runes to me.

I'm glad Rust feels more like Ruby and Python and that method and field names are legible.

My eyes just glaze over:

    UPB_API_INLINE
    const struct upb_MiniTableField* upb_MiniTable_FindFieldByNumber(
        const struct upb_MiniTable* m, uint32_t number) {
      const uint32_t i = number - 1;  // 0 wraps to UINT32_MAX
    
      // Ideal case: index into dense fields
      if (i < m->UPB_PRIVATE(dense_below)) {
        UPB_ASSERT(m->UPB_ONLYBITS(fields)[i].UPB_ONLYBITS(number) == number);
        return &m->UPB_ONLYBITS(fields)[i];
      }
    
      // Early exit if the field number is out of range.
      uint32_t hi = m->UPB_ONLYBITS(field_count);
      uint32_t lo = m->UPB_PRIVATE(dense_below);
      UPB_ASSERT(hi >= lo);
      uint32_t search_len = hi - lo;
      if (search_len == 0 ||
          number > m->UPB_ONLYBITS(fields)[hi - 1].UPB_ONLYBITS(number)) {
        return NULL;
      }
    
      // Slow case: binary search
      const struct upb_MiniTableField* candidate;
    #ifndef NDEBUG
      candidate = UPB_PRIVATE(upb_MiniTable_ArmOptimizedLowerBound)(
          m, lo, search_len, number);
      UPB_ASSERT(candidate ==
                 UPB_PRIVATE(upb_MiniTable_LowerBound)(m, lo, search_len, number));
    #elif UPB_ARM64_ASM
      candidate = UPB_PRIVATE(upb_MiniTable_ArmOptimizedLowerBound)(
          m, lo, search_len, number);
    #else
      candidate = UPB_PRIVATE(upb_MiniTable_LowerBound)(m, lo, search_len, number);
    #endif
    
      return candidate->UPB_ONLYBITS(number) == number ? candidate : NULL;
    }