Performance

What are the typical file transfer speeds I can expect?

Test results

The following table shows the typical transfer speeds on various different Client CPUs, using both sync and async FTP clients.

Client CPU: Core I9-13900K

FluentFTP Class	Application	Download speed	Upload speed
FtpClient	WinForms	111 MB/sec	80 MB/sec
AsyncFtpClient	WinForms	111 MB/sec	80 MB/sec
FtpClient	Console	111 MB/sec	80 MB/sec
AsyncFtpClient	Console	60 MB/sec	80 MB/sec

Client CPU: Core I7-6700HQ

FluentFTP Class	Application	Download speed	Upload speed
FtpClient	WinForms
AsyncFtpClient	WinForms
FtpClient	Console
AsyncFtpClient	Console

Test conditions

These are the test conditions that were used to measure the file transfer speed.

• Network transfer rate: 1Gb/s
• Server side CPU is one core of a Core I9-10900K writing to / reading from a 1500MB/s SSD.
• Server is proFTPd on Debian 11.
• Client side, unless otherwise stated is Windows 10, writing to / reading from a 1500MB/s SSD.
• Downloading a 1GB file filled with random bytes.
• Winforms or Console Application using FluentFTP V50+, Release type build, .NET 6
• Using SslStream (Performance of FluentFTP.GnuTLS was equivalent in all cases)

What aspects influence FTP file transfer speed?

1. Network speed/latency

While the influence of network transfer speed is a major factor in the performance of large data transfers, the transfer of very many small files will profit also from overall network latency and round-trip times, as the command-response sequences on the control connection become a bottleneck.

2. Server CPU / IO performance

The server must of course be able to supply or ingest data fast enough fully utilize the available network resources.

3. Client CPU / IO performance

The client must also not be hampered by a slow or busy CPU whilst the IO performance will seriously slow down transfers.

Does FTP slow down file transfer?

No, FTP(S) was designed to have virtually no protocol overhead on the data transfer itself. No overhead, that is, other than that imposed by Ethernet, as well as IP and TCP.

What is the theoretical maximum file transfer speed?

Theoretical maximum transfer rate on a 1Gb/s ethernet connection, taking all ethernet frame headers into account would be 118MB/sec (even 123MB/s using jumbo frames = 99% of 1Gb/s).

Now take into account IP / TCP headers, which can be minimised by avoiding fragmentation, using jumbo packets, avoiding resends, delaying ACKs and other measures. This can be estimated at about 2.8% of the available bandwidth, so we arrive at a speed of about 114MB/sec.