Use WinSCP, configure it to save the config to an ini file and change the background transfer count to like 20-30. It won't speed up single file transfer, but will speed up multi file transfers. And you can always put any content uploadable to a seedbox to a 7z archive split every 500MB and then extract it on the seedbox that way getting access to the multi file upload speed even if you only want to upload a single file. To speed up the archivation process you can set the compression level to 0 - Store so it would just split the file instead of compressing it, which is significantly faster than the default compression level.