Help mi please with optimalization of simple Python code

[Chrabros]

Hello,,
I have written my first simple application in Python and I have a
problem that processing of cca 10MB file takes ages (30 seconds).
I think that problem is in data structure used.
Could you please give me an advice how to optimize the following
piece of code?
Thank you in advance.

 Dalibor


while y<maxY :
  x=0
  line = file.read(maxX*2)
  while x<maxX :
    val   = ord(line[x*2+2])*256 + ord(line[x*2+3])
   ... here are some contditions which count the occurences 
       of unique  values of val
    x=x+2
y=y+2

I think that main problem is in usage of string "line" but I
do not know how to it different way.