It'll be faster if you copy the string into memory and then use SYS(2600) to read byte by byte.

LOCAL lnX, lcString
lcString = REPLICATE("1234567890",100000)

	Declare Integer HeapAlloc in Win32Api Integer, Integer, Integer
	Declare Integer HeapFree in Win32APi Integer, Integer, Integer
	Declare Long GetProcessHeap in Win32API
      
lnLength = LEN(lcString)
TRANSFORM(lnLength,"9,999,999")

IF .T.
lnSecs = SECONDS()

lnBase = HeapAlloc( GetProcessHeap(), 0, m.lnLength )
Sys(2600, m.lnBase, m.lnLength, m.lcString)
FOR lnX = 1 TO lnLength
	lcVal = Sys(2600,m.lnBase-1+m.lnX,1)
ENDFOR
HeapFree( GetProcessHeap(), 0, m.lnBase )

? "SYS(2600): " +  TRANSFORM(SECONDS() - lnSecs )

ENDIF

That's 0.33 seconds on my machine vs 37 seconds with SUBSTR().