为什么Matlab在for循环中迭代次数慢但在迭代次数少时又快呢？

我正在运行一个功能，以从100,000多个患者X射线dicom文件中提取一些信息。为了安全起见，这些文件存储在veracrypt加密容器中。

当我在文件的一小部分上运行该函数时，它的执行速度非常快，但是，当我在整个数据集上运行该函数时，它的运行速度却很慢，从每秒几个文件变为每秒一个文件（大约）。

我在徘徊为什么会这样？我曾尝试将数据存储在ssd和普通硬盘上，并且在使用较大的数据集而不是较小的数据集时会出现相同的速度变慢。

我添加了以下代码以供参考，但尚未对其进行完整注释。.这是针对我的论文的，所以一旦提取完成，我会做的。

谢谢你的帮助。

function [ DB, corrupted_files ] = extract_from_dcm( folder_name )
%EXTRACT_FROM_DCM Summary of this function goes here
%   Detailed explanation goes here

if nargin == 0
    folder_name = 'I:\Find and Treat\MXU Old Backup\2005';
end

Database_Check = strcat(folder_name, '\DataBase.mat');

if exist(Database_Check, 'file')

    load(Database_Check);
    entry_start = length(DB) + 1;

else

    entry_start = 1;
    [ found_dicoms ] = recursive_search( folder_name );

end

mat_file_location = strcat(folder_name, '\DataBase.mat');
excel_DB_file = strcat(folder_name, '\DataBase.xlsx');
excel_Corrupted_file = strcat(folder_name, '\Corrupted_Files.xlsx');

% the recursive search creates a struct with the path for each
% dcm file found. the list is then recursivly used to locate
% the image and extract the relevant information from it.


fprintf('---------------------------------------------\n');
fprintf('Start Patient Data Extraction\n');
tic
h = waitbar(0,'','Name','Patient Data Extraction');
entry_end = length(found_dicoms);

if entry_end == 0
    %     set(handles.info_box, 'String', 'No Dicom Files Found in this Folder or its Subfolders');
else
    %     set(handles.info_box, 'String', 'Congratulations Dicom Files have been found Look Through the Data Base using the Buttons Below....Press Save Button to save the Database. (Database Save format is EXCEL SpreadSheet and MAT file');

    for  kk = entry_start : entry_end
        progress = kk/entry_end;
        progress_percent = round(progress * 100);
        waitbar(progress,h, sprintf('%d%%   %d/%d of images processed', progress_percent, kk, entry_end));

        img_full_path = found_dicoms(kk).name;

        %         search_path = folder_name;
        %         img_full_path = strrep(img_full_path, search_path, '');

        try              %# Attempt to perform some computation
            dicom_info = dicominfo(img_full_path); %# The operation you are trying to perform goes here

            try              %# Attempt to perform some computation
                dicom_read = dicomread(dicom_info); %# The operation you are trying to perform goes here

                old = dicominfo(img_full_path);

                DB(kk).StudyDate = old.StudyDate;
                DB(kk).StudyTime = old.StudyTime;


                if isfield(old.PatientName, 'FamilyName')
                    DB(kk).Forename = old.PatientName.FamilyName;
                else
                    DB(kk).Forename = 'NA';
                end

                if isfield(old.PatientName, 'GivenName')
                    DB(kk).LastName = old.PatientName.GivenName;
                else
                    DB(kk).LastName = 'NA';
                end

                if isfield(old, 'PatientSex')
                    DB(kk).PatientSex = old.PatientSex;
                else
                    DB(kk).PatientSex = 'NA';
                end

                if isempty(old.PatientBirthDate)
                    DB(kk).PatientBirthDate = '00000000';
                else
                    DB(kk).PatientBirthDate = old.PatientBirthDate;
                end

                if strcmp(old.Manufacturer, 'Philips Medical Systems')
                    DB(kk).Van = '1';
                else
                    DB(kk).Van = '0';% section to represent organising by different vans
                end

                DB(kk).img_Path = img_full_path;

                save(mat_file_location,'DB','found_dicoms');

            catch exception  %# Catch the exception
                fprintf('read - file %d corrupt.\n',kk);
                continue       %# Pass control to the next loop iteration
            end



        catch exception  %# Catch the exception
            fprintf('info - file %d corrupt.\n',kk);
            continue       %# Pass control to the next loop iteration
        end


    end

end


[ corrupted_files, DB ] = corruption_check( DB, found_dicoms, folder_name );


toc
fprintf('End Patient Data Extraction\n');
fprintf('---------------------------------------------\n');

fprintf('---------------------------------------------\n');
fprintf('Start Saving Extracted Data \n');
tic


save(mat_file_location,'DB','corrupted_files','found_dicoms');

if isempty(DB)
    msg = sprintf('No Dicom Files Found');
    msgbox(strcat(msg));
else
    DB_table = struct2table(DB);
    writetable(DB_table, excel_DB_file);
end

close(h);

toc
fprintf('End Saving Extracted Data \n');
fprintf('---------------------------------------------\n');

end