Segmented SFTP Downloads Explained: Why One Connection Is Slow and How to Fix It

A segmented download over SFTP splits one large file into pieces, pulls each piece over its own SSH connection at the same time, and writes them all into the same file. It is the biggest speed fix for anyone pulling big files from a distant seedbox, because one SFTP connection is capped by the round trip to the server, not by your line. Eight connections on the same file lift that cap eight times, until your line or disk is full.

evoftp, our free SFTP client, does this for every file by default with nothing to switch on. Here is why one connection is slow, what segmenting costs, and how to turn it on in each client.

Why one SFTP connection is slow

Every connection can only have a limited amount of data “in flight” — sent by the server but not yet acknowledged by your computer. That limit is the window. When it is full, the server waits one full round trip for your acknowledgement before sending more.

So one connection moves at most window size divided by round-trip time. Networking books call this the bandwidth-delay product; in plain words it is a bucket you can only refill once per round trip. A 1 MiB window over a 0.4 s round trip is about 2.5 MB/s, whatever your line can carry.

That is what we measured. In our tests a Mac in India connecting to a European seedbox at 250–700 ms round trip got about 2.4 MB/s per connection at a 1 MiB window, on a line that could carry far more. Nothing was broken; the connection spent most of its time waiting for acknowledgements to cross the planet. WinSCP’s own FAQ on slow transfers names the same causes — single-core encryption and SFTP latency — and points to parallel connections as the fix.

How splitting the file across connections helps

You cannot shorten the round trip, and the window can only grow so far. What you can do is run several windows at once. Split a 4 GB file into four 1 GB ranges, open four SSH connections, and give each its own range. Each is still capped at ~2.4 MB/s, but there are four, so the file arrives at ~9.6 MB/s; with eight, ~19 MB/s. This scales until something else binds:

• Your line. Once the segments together fill your bandwidth, more add nothing.
• Your disk. Segments write to the same file at once; a slow HDD or USB stick caps you first.
• CPU on either end. Every SSH connection encrypts and decrypts its own stream.

evoftp adds two refinements: the window is sized from the measured ping (up to 16 MB in the Long distance profile), and segments that finish early steal work from lagging ones. The same Mac in India went from ~2.4 MB/s per connection to line speed with the 16 MB window and 8 segments.

What segmented downloads cost

• More SSH sessions. Each segment is a full handshake and login, so clients cap total connections; evoftp’s Balanced profile is 4 segments per file, 3 files at once, 8 connections.
• CPU for encryption. CuteFTP’s documentation says multi-part transfers are not recommended over SSH or SSL for this reason. Rarely the limit on a modern laptop.
• Servers that cap connections per IP. CuteFTP’s docs warn multi-part transfers break on such servers; FileZilla’s wiki documents the symptom as error 421 “Too many connections”; Cyberduck notes that segmenting needs server support for concurrent connections.
• A merge step, in some clients — see below.

Two ways to implement it: merge, or write in place

Temp parts, then merge. Cyberduck downloads each segment to its own temporary file and joins them afterwards. The merge is a second pass over the whole file — Cyberduck’s own docs say it “may take a considerable amount of time” and only works if all parts are downloaded — and it needs disk space for the parts plus the result.

Write in place. lftp pget and evoftp seek each connection to its byte range in one pre-sized file and write straight there. No merge, the file is complete the moment the last segment lands, and progress survives an interruption: evoftp keeps a small <file>.evoftp-part sidecar recording which ranges are done, so re-downloading the same file into the same folder continues where it stopped. That is the mechanism behind SFTP resume.

Which clients support segmented download SFTP transfers

From each vendor’s own documentation, or for FileZilla its developers’ forum answers.

How to turn it on

evoftp

Nothing to turn on. Settings → profile: Balanced (4 segments), Long distance (8, 16 MB window), Fast fiber (2), Gentle (1) or Custom (segments 1–8, connections 2–12). Per file, use the arrow beside Download for 8/4/2/1. Downloads: neo.myseedbox.site/share/evoftp/.

lftp

lftp sftp://[email protected]
pget -n 8 -c "downloads/Some.Large.File.mkv"
mirror --parallel=3 --use-pget-n=8 downloads/Some.Folder

Cyberduck

Already on. Preferences → Transfers → General → Downloads → “Segmented download with multiple connections for file”. The connection limit in the Transfers window also caps segments.

WinSCP

Preferences → Background → “Use multiple connections for single transfer”, then “Use multiple connections for single file” and a size threshold. SFTP sessions only.

CuteFTP

File → Download Advanced → Multi-part Download, then HIGH or MAX. Built for FTP; the docs discourage it over SFTP.

FileZilla

You cannot. Raising simultaneous transfers to 10 helps a folder of many files, not one large file — which is why FileZilla feels slow from a seedbox. The fix is a different client.

Frequently asked questions

Is segmented downloading allowed on my seedbox?

Yes. Your seedbox accepts SFTP on port 22 and a client opening several sessions with your login is normal. evoftp’s Balanced profile (8 connections) is what we ship and test against our own servers, and lftp is installed on every box. 8–12 connections is plenty; past line speed there is nothing to gain.

How many segments should I use?

Start with 4 and go up only if the transfer sits well below your line speed. Long round trips (250 ms and up) benefit from 8. On a short hop, 2 is enough. Once your line or disk is full, extra segments add only CPU and SSH sessions.

Does it help uploads?

The same maths applies, but few clients segment uploads. evoftp uploads on a single stream. CuteFTP’s multi-part upload needs the server to support a COMB command to join the parts, which a standard SFTP server does not. On a seedbox nearly all traffic is downloads, so this rarely matters.

Does it help on a LAN or a nearby server?

Barely. At a 1 ms round trip, one 1 MiB window already allows around 1 GB/s, so disk and CPU take over as the limit. CuteFTP’s documentation says the same: multi-part transfers are ineffective on a LAN. Our LAN-adjacent test moved 1.07 GB in 4.9 s, but that was disk and encryption speed, not the segment count.

Next step

If downloads from your seedbox crawl at a few MB/s while a speed test says you have far more, you have a latency-capped single connection. Install evoftp, connect with the connection file from your client area, and download the same file again — segmented by default, no settings to find. For every other way to move files off a seedbox, see how to transfer files from your seedbox.