[O] long code blocks making Org Mode very slow

[Angus M]

I'm trying to run a long Python code block (~4000 lines) in org mode.  The
code is sandwiched between:
#+begin_src python
<python code block>
#+end_src

It runs fine, but org mode becomes incredibly slow - when typing text there
is about a 10 second delay before the character appears on the screen.  If I
reduce the number of lines of code in the block there is a noticeable
increase in speed.

I am not using line numbers, and the delay is not ameliorated by turning off
fontification.

If I add a semi-colon before #+begin_src, to comment it off (get rid of src
environment), then org mode returns to it's normal speed.

Do I just have to accept that long code blocks can't be entered into Org Mode?  

Any help would be appreciated.  I am running Org Mode (version 8.2.10) on
Windows 8.

Thanks

Angus M