As you are using Windows and an older version of MATLAB, we recommend that you switch from using xlswrite() to using the File Exchange contribution xlswrite1() which is more efficient. In some of the older releases around the time of yours, xlswrite() did not reliably release resources and so would slow down when it was called repeatedly.
If your code is repeatedly writing to the same file, then xlswrite1() will help, but better yet would be to create a cell array that you made all of the changes into and then only xlswrite1() the final version.