getting execution time more than serial, procedure to optimise the following code using either spmd or using parfor loop

Hi, Following code uses structure array , i have parallelised the code as follows but execution time is more compared to serial. if (nargin<3 isempty(bandList)) bandList=10:11; end

      % making sure band numbers are not repeated.
      opBandList=unique(bandList);

      % Removing any band other than 10 and 11
      opBandList=[any(opBandList==10)*10 any(opBandList==11)*11];

      % checking if the requested bandNumber is loaded.
      bandIsLoaded=cellfun(@(x) ~isempty(x),Data.Band);
      bandIsLoaded=bandIsLoaded(opBandList);
      if (all(~bandIsLoaded))
        error('None of the requested bands are loaded')
      elseif (any(~bandIsLoaded))
        disp('The following Bands are not loaded:')
        disp(opBandList(~bandIsLoaded))
        disp('These bands are ignored.')
      end
      opBandList=opBandList(bandIsLoaded);

     output.SatBT_bandList=opBandList;     
      output.SatBT=cell(numel(opBandList));
 parpool('local',4);
spmd
     for i=1:numel(opBandList)
 output=struct();
        m=Data.MetaData.L1_METADATA_FILE.RADIOMETRIC_RESCALING.(['RADIANCE_MULT_BAND_' num2str(opBandList(i))]);
        b=Data.MetaData.L1_METADATA_FILE.RADIOMETRIC_RESCALING.(['RADIANCE_ADD_BAND_' num2str(opBandList(i))]);
        K1=Data.MetaData.L1_METADATA_FILE.TIRS_THERMAL_CONSTANTS.(['K1_CONSTANT_BAND_' num2str(opBandList(i))]);
        K2=Data.MetaData.L1_METADATA_FILE.TIRS_THERMAL_CONSTANTS.(['K2_CONSTANT_BAND_' num2str(opBandList(i))]);
        tmpOutput=m*double(Data.Band{opBandList(i)})+b;
        tmpOutput=K2./ log(K1./tmpOutput +1.0 );
        tmpOutput(Data.Band{opBandList(i)}==0)=NaN;
        output.SatBT{i}=tmpOutput;       
     end