EK-TM4C123GXL: slower execution time from RAM

connecting a scope to a GPIO output pin I can measure the time its takes
for a loop to repeat. The loop simply toggles the GPIO pin. The clock rate
on the Tiva C series launchpad is documented as 80 MHz (12.5 ns cycle).

Running the code from ROM I measured a pulse width of 100ns (8
cycles). From RAM I get 125ns instead (10 cycles).

Why running from RAM is slower in this case? I was actually expecting the
opposite, a faster RAM execution.

Thanks,
 Rodrigo

from ROM execution

00000000000002b0 0000001c <loop()>:
 2b0:    ldr    r3, [pc, #20]    ; (2c8 <loop()+0x18>)
 2b2:    ldr    r2, [r3, #4]
 2b4:    ldrb    r1, [r3, #21]
 2b6:    movs    r3, #0
 2b8:    str.w    r3, [r2, r1, lsl #2]
 2bc:    ldr.w    r3, [r2, r1, lsl #2]
 2c0:    mvns    r3, r3
 2c2:    str.w    r3, [r2, r1, lsl #2]
 2c6:    b.n    2bc <loop()+0xc>
 2c8:    .word    0x2000026c

from RAM execution

200002b0:    ldr    r3, [pc, #20]    ; (200002c8 <loop()+0x18>)
200002b2:    ldr    r2, [r3, #4]
200002b4:    ldrb    r1, [r3, #21]
200002b6:    movs    r3, #0
200002b8:    str.w    r3, [r2, r1, lsl #2]
200002bc:    ldr.w    r3, [r2, r1, lsl #2]
200002c0:    mvns    r3, r3
200002c2:    str.w    r3, [r2, r1, lsl #2]
200002c6:    b.n    200002bc <loop()+0xc>
200002c8:    .word    0x20000a6c