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🔄 Syncthing First Sync

Syncthing First Sync Is Slow Do the Big Folder Over the LAN First

Syncthing is fast at daily changes and brutally slow at one thing: the very first sync of a huge folder between devices in different places. The cure costs nothing — do the first sync at home, on the LAN, before the devices ever part.

✍️ Technical Anshu🖥️ Windows / Linux⏱️ Saves hours on any large folder
🎥 The main Syncthing video shows a first sync completing — this guide covers the big folder strategy

Daily sync moves what changed: a document here, three photos there. It is light work. The first sync moves everything — every photo since 2019, every project archive, every video. If your devices meet for the first time across the internet, that mountain must climb through your home connection's upload speed, which on most plans is a fraction of the download speed people quote. A 200 GB photo library can honestly take days. The same library, device to device on your own LAN, moves in an evening. This guide is the discipline of arranging that.

The same 200 GB folder — two very different first syncs (illustrative figures)
OVER THE INTERNET (first sync, devices apart) Ceiling ......... your home UPLOAD speed Typical result .. 200 GB crawls for 1 to 3 days Risk ............ router reboots, sleep, ISP hiccups all slow an already long job ON THE LAN (first sync, same room, wired if possible) Ceiling ......... your network, often 10x faster Typical result .. the same 200 GB in an evening Bonus ........... you watch it finish and verify before the devices ever separate After the first sync, only changes travel — and changes are small, wherever the devices are.
Illustrative panel — real times depend on your disks, network and plan
1

Why the First Sync Is a Different Animal

Three things make the first sync heavy, and none of them are faults:

  1. Everything is "new". Syncthing scans the folder, hashes every file to build its index, then transfers the full content of each file to devices that have none of it.
  2. Hashing takes time on big media. Hundreds of gigabytes of video must be read end to end on the source device before and while it sends. An old hub CPU shows here.
  3. The path decides the ceiling. Across the internet, your upload speed caps everything, and the receiving device's patience — sleep settings, Wi-Fi drops — interrupts a job measured in days.

After this one mountain, Syncthing returns to what it is good at: noticing the twelve files that changed today.

2

The LAN-First Routine

Whenever a device joins or a big folder is first shared, do this:

  1. Bring the devices together. Same home or office network. Plug the hub into the router with a cable if you can; wired beats Wi-Fi for a bulk transfer, and plug laptops into power so nothing sleeps mid-job.
  2. Pair and share as normal. Confirm in the interface that the connection type shows a local address — that proves traffic is on the LAN, not wandering out to the internet and back.
  3. Share one big folder at a time. Let it reach Up to Date before adding the next monster. Watching one folder complete is how you learn your real speeds.
  4. Turn off sleep for the evening. A laptop that sleeps at 40% is not broken — it is obedient. Change the setting, or keep it awake, until the first sync lands.
  5. Verify before separating. Folder state "Up to Date" on both sides, and open a few files on the receiving device. Only then let the devices go their separate ways.
3

If Devices Cannot Meet — Honest Fallbacks

Sometimes the laptop is already in another city. Workable options, in order:

  1. Seed by external drive. Copy the folder onto a USB drive, carry or courier it, place the copy in the right path on the receiving device before sharing the folder. Syncthing scans, recognises the files by hash, and transfers only what differs. The mountain crosses the country in a bag instead of through an upload pipe.
  2. Accept a staged internet sync. Share the folder and let it run for days in the background. It will finish; Syncthing resumes automatically after every interruption. Just do not judge the tool by this one job.
  3. Prioritise. Share the small working folders first so the device is useful today, and let the archive folder trickle in behind them.
4

Small Print That Saves Annoyance

  1. Free disk space matters on the hub. A first sync lands the full folder plus, if versioning is on, future versions. Check space before starting, not at 97%.
  2. Phones: do the camera folder's first sync at home on Wi-Fi, charging. Mobile data and a hot phone throttle the worst possible first job.
  3. "Syncing 99%" for a long time usually means a few large files remain, or one device is rescanning. Patience, plus a look at which file is in flight, beats restarting anything.
🔴 The habit in one sentence: devices shake hands over the internet, but they move house over the LAN. Every time you add a device or a huge folder, put the devices in the same room for the first sync — your future self, watching daily changes fly across the country in seconds, will be glad the mountain was moved at home.

🎥 Watch a First Sync Complete in the Video

The main Syncthing setup video shares real folders between three devices and shows them reaching Up to Date.

Watch Full Video
Frequently Asked Questions

Syncthing shows the folder syncing but the time keeps growing. Is it stuck?

Usually not. For very large files the remaining time estimate swings as speeds vary, and the last few percent of a huge first sync genuinely takes a while. Check that bytes are still moving and that the connection is a local one; steady movement, however slow, means patience is the fix.

Can I pause a big sync and resume it later?

Yes. You can pause a folder or a device in the interface, and Syncthing also resumes automatically after reboots, sleep and network drops. It continues from what it already has — nothing restarts from zero.

Does the seeding trick with a USB drive really avoid re-downloading everything?

Yes, if the files are identical. Syncthing identifies content by cryptographic hash, not by name or date. Place the copied folder at the exact path you will share, then share it — the scan finds matching hashes and only true differences cross the network.

This guide is part of the Syncthing series by Technical Anshu • Watch the main setup video for the full live build 🙏