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Slow Transfer Speeds Using HTTPS  -  KB Article #1796

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In 2008 with the release of Serv-U 7, RhinoSoft introduced multi-threaded processing to the Serv-U File Server. This new design and architecture allows Serv-U to process bandwidth and CPU-intensive operations separately from the main server thread, improving efficiency and in many cases increasing transfer speeds.

When transferring files using the HTTPS protocol these improvements may not be evident because when compared to HTTP, FTP, FTPS and SFTP, HTTPS transfers can seem far slower. Research into this difference in transfer speeds showed unexpected results - many of the commonly used web browsers (Firefox, Internet Explorer and more) transfer files in extremely small packets, ranging from 4096 bytes to 8192 bytes. This is a drastic difference when compared to the over 500,000 bytes typically sent in a single FTP/FTPS/SFTP/HTTP transfer packet. Using packets of only 4096 bytes instead of 500,000 bytes produces over 120 times the overhead and processing requirements, slowing transfers down by a significant amount.

If at some point mainstream browsers are modified to send larger transfer packets for HTTPS transfer speeds in Serv-U will drastically increase - until that point the only workaround for this browser limitation is to transfer files with HTTP or by a different protocol such as FTP, FTPS, or SFTP.