It was still nowhere near as fast as a good software sequence on Nehalem or Sandybridge. The startup cost to the microcode engine was simply too high, and its

It was still nowhere near as fast as a good software sequence on Nehalem or Sandybridge. The startup cost to the microcode engine was simply too high, and its handling of misalignment was very inefficient. For "small" (< 1K) unaligned buffers (which are actually a large portion of memcpy usage on a live system), good software sequences were 3-4x faster than REP MOVS. It did perform competitively for large all-aligned copies, which are what most people tend to look at when they post "memcpy benchmarks", but those turn out to be a relatively small portion of actual usage in most workloads.