w32 redisplay lags much more seriously when editing big files.

[Kevin Yu]

Hi, All
     The performance issue of new backend on w32 has been talked a lot on this list. If you hold down the "C-n", you will see half of the window is blank when the cursor goes over the bottom line of window. I have tried it on a big file with 20000 lines. It lags much more in a big file than a small file (about 2000 lines). So the file size will affected redisplay time?
     I wish it'll help you guys to improve the performance on w32.
     Best wishes.

Kevin