These 8086 instructions were pretty cool coming from a 6502 background. The usual way to copy a block of memory in 6502 was something like this:

    ldy #len
  loop:
    lda source-1,y   ; memory accesses: loads opcode, address, value
    sta dest-1,y     ; memory accesses: loads opcode, address, stores value
    dey              ; memory accesses: loads opcode
    bne loop         ; memory accesses: loads opcode, offset

So 4 instructions and a minimum of 9 memory accesses per byte copied, more if not in the zero page. Even unrolling the loop gets you down to 6.

Compare this to the 8086:

      mov cx, len
      mov si, source
      mov di, dest
  rep movsb            ; memory accesses: read the opcodes, then 1 load and 1 store per byte

Even forgetting about word moves, you're down to 2 memory accesses per byte. No instruction opcode or branching overhead.