Half of a 6 GB backup file uploaded, the Wi-Fi blips, and the transfer dies. Does SFTP make you start from zero? The honest answer is: it depends on the client - and the good ones handle it well.
SFTP itself is reliable - every byte is checked inside an encrypted, integrity-protected SSH connection, so a file that arrives is a file that is intact. The weak point in big transfers is never the protocol; it is interruptions: sleep, Wi-Fi drops, a router reboot. Resume support is a feature of the client program, not the server, and the difference between clients decides whether a broken transfer costs you seconds or the whole file again.
When a transfer breaks, a partial file sits on the server. A client with resume support compares the size of that partial file with the original and continues appending from exactly where it stopped. In FileZilla, reconnect, start the same transfer and choose Resume when it asks about the existing file - never Overwrite, which restarts from zero. WinSCP offers resume for larger files and can reconnect automatically after a drop. On the command line, the built-in client's reput command resumes an interrupted upload and reget resumes a download.
Resume and reliability are only proven by a check. Compare the file size first - it must match exactly. For important files, compare a hash: run certutil -hashfile backup.zip SHA256 on the PC that has the original and again where the copy landed. Identical hashes mean the transfer is byte-for-byte perfect, whatever happened along the way. This thirty-second habit is what separates a hopeful copy from a verified one.
Encryption costs a little speed. On an old, slow CPU an SFTP transfer can run somewhat below plain FTP or a local file share, because every byte passes through SSH encryption. On any reasonably modern PC the network is the limit, not the encryption. That small cost buys you passwords and data that nobody on the path can read - for anything that matters, it is the right trade, and it is exactly why offices moved from FTP to SFTP.
See the complete SFTP server setup on screen, step by step, in the Technical Anshu video.
Watch Full VideoThe SSH connection checks the integrity of the data as it travels, so silent corruption in transit is effectively prevented. For high-value files, still compare size and a SHA256 hash with certutil on both sides - that also covers any problem before or after the transfer itself.
Your client is overwriting instead of resuming, or it has no resume support at all. In FileZilla choose Resume when prompted about the existing file; in WinSCP enable resume for large files. Basic phone apps often lack resume entirely - switch to a proper client for big files.
On a wired LAN, yes - it will generally run at network speed on a modern PC. The practical limits are Wi-Fi quality, disk speed and interruptions, not the protocol.