The default behavior of an FTP export is to read and process each file one at a time. That is, the export stops retrieving files until the current file is completed. If you are exporting several large files, particularly in XML format, the transfer can proceed much more slowly than expected.
You can control the speed and performance of FTP file transfers in the Advanced > Batch size setting of the export. It lets you specify how many concurrent files you want processed in a single request (up to 1,000).
Setting a maximum number of files allows you to optimize your flows for large files and avoid timeout errors. Then, for smaller files, processing a greater number of files in a single batch keeps the flow more performant.