Your best bet will be to build an array to write the column in one call; the MATLAB functions open/close Excel for each call so trying to write single cells in a tight loop is abadidea™.
Alternatively, and the approach I take on something somewhat similar is to read the template into a MATLAB table, modify the pieces of the table as needed, then simply rewrite the whole thing with writetable. The other functions above actually convert the input data to a table and call writetable with appropriate parameters, anyways.
