Hello Alessandro Marzini.

> LOOP
>         movlw   0xFF
>         movwf   PORTB

2 clocks are carring out

>         <delay routine>
>
>         clrf    PORTB

1 clock is carring out and zero flag in STATUS is set


>         <delay routine>
>
>         goto    LOOP

Carefully check your delay routine code. Probably you test zero
flag within it and second case with zero on input result to less/
/greater delay. For the other hand you may substitute second

        clrf    PORTB

with
        movlw   0
        movwf   PORTB

and check the result.

WBR Dmitry.

PS. Using bsf/bcf instructions on real i/o port is definitly not
    recommended. In situation with mixing in and out pins on the
    same port it may result to writing to port undesirable value.
    This also related to any Read/Modify/Write_back commands.
    It will be better to use shadow register and only write to
    port output value with reading from shadow.