Changed it slightly and still the results are not drastically different in VFP8:

DO WHILE: 00:00:49
FOR/ENDFOR: 00:00:46

CLEAR
SET TALK OFF
SET SAFETY OFF


CREATE TABLE mytable (nvalue n(5,0), cvalue c(10))
SELE mytable
FOR i = 1 TO 50000
	APPEND BLANK
	REPLACE mytable.nvalue WITH i
ENDFOR

SELE mytable
GO TOP
mval = 0
? "DO WHILE LOOP"
? "START1:"
time1 = DATETIME()
?? time1
DO WHILE !EOF()
	WAIT WINDOW "Processing "+ALLTRIM(STR(mval))+" of 50,000" NOWAIT
	DO countit WITH mval
   REPLACE mytable.nvalue WITH nvalue+1
	IF EMPTY(mytable.cvalue)
		REPLACE mytable.cvalue WITH "TEST1"
	ELSE
		REPLACE mytable.cvalue WITH "TEST2"
	ENDIF
	IF !CHECKIT(RECNO())
		EXIT
	ENDIF
	IF mval > 999998
		EXIT
	ENDIF
   SKIP
ENDDO
time2 = DATETIME()
timegone = timepassed(time1,time2)
? "FINISH:"
?? time2
? "TIME ELAPSED: "
?? timegone

?
?
?

SELE mytable
GO TOP
mval = 0
? "FOR ENDFOR"
? "START2:"
time1 = DATETIME()
?? time1
FOR i = 1 TO RECCOUNT()
	WAIT WINDOW "Processing "+ALLTRIM(STR(mval))+" of 50,000" NOWAIT
	DO countit WITH mval
	REPLACE mytable.nvalue WITH nvalue + 1
	IF EMPTY(mytable.cvalue)
		REPLACE mytable.cvalue WITH "TEST1"
	ELSE
		REPLACE mytable.cvalue WITH "TEST2"
	ENDIF
	IF !CHECKIT(RECNO())
		EXIT
	ENDIF
	IF mval > 999998
		EXIT
	ENDIF
	SKIP
ENDFOR
time2 = DATETIME()
timegone = timepassed(time1,time2)
? "FINISH:"
?? time2
? "TIME ELAPSED: "
?? timegone


FUNCTION checkit
LPARAMETERS recordno
IF RECNO() <> recordno
	GOTO recordno
ENDIF
IF EMPTY(mytable.nvalue)
	? RECNO()
ENDIF
RETURN .T.

PROCEDURE countit
LPARAMETERS newval
newval = newval + 1
RETURN newval

PROCEDURE timepassed
LPARAMETERS starttime, endtime
timetaken = endtime - starttime
x = ttoc( { 00:00 } + timetaken, 1)
x = substr(x,9,2) + ':' + substr(x,11,2) + ':' + right(x,2)
RETURN x