Switching from SUB to SBB changes runtime by 15%. Can you explain why?

sub.asm

; Minimal example, see also http://stackoverflow.com/q/26266953/3766665
; To build (Linux):
;   nasm -felf64 func.asm
;   ld func.o
; Then run:
;   perf stat -r10 ./a.out
; On Haswell and Sandy Bridge, observed runtime varies 
; ~15% depending on whether sub or sbb is used in the loop
section .text
global _start
_start:
  push qword 0h       ; put counter variable on stack
  jmp loop            ; jump to function
align 64              ; function alignment.
loop:
  mov rcx, 1000000000
align 64              ; loop alignment.
l:
  mov rax, [rsp]
  add rax, 1h
  mov [rsp], rax
; sbb rcx, 1h         ; which is faster: sbb or sub?
  sub rcx, 1h         ; switch, time it, and find out
  jne l               ; (rot13 spoiler: foo vf snfgre ol 15%)
fin:                  ; If that was too easy, explain why.
  mov eax, 60
  xor edi, edi        ; End of program. Exit with code 0
  syscall