merging files take too long time

Question

In my app i implement a download manager that using multi connection for download a file and save content in temp files and finally when download completed merge these files in a single file.
My problem is the last step (merging files) take too long to be completed even more than downloading the file(for example a file with 250 Kb downloaded in 5 second and merged in about 10 - 15 second). Im using this method for merging temp file :

private void mergeAndActivate() {
    new AsyncTask<Void, Integer, Void>() {
        @Override
        protected void onPreExecute() {
            // only toast a message
        }

        @Override
        protected Void doInBackground(Void... params) {
            long leninfile=0, leng=0;
            int count=0, data=0;
            try {
                String filePath =download.getFile().getPath();
                File filename = new File(filePath);
                RandomAccessFile outfile = new RandomAccessFile(filename,"rw");
                while(true) {
                    filename = getTempFile(count);
                    if (filename.exists()) {
                        RandomAccessFile infile = new RandomAccessFile(filename,"r");
                        data=infile.read();
                        while(data != -1) {
                            outfile.write(data);
                            data=infile.read();
                        }
                        leng++;
                        infile.close();
                        count++;
                    } else break;
                }
                outfile.close();
            } catch(Exception e) {
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void aVoid) {
            super.onPostExecute(aVoid);
            deleteTempFiles();

            // Toast a mesage

        }



    }.execute();
}