SMBDrop

Why iPhone SMB transfers are slow, and how to speed them up

By Isaac Griffiths, developer of SMBDrop ·

You've got a NAS that copies at full speed from your computer, and then the iPhone crawls. A folder of videos that should take minutes takes an hour. It's one of the most common complaints about SMB on iOS, and there isn't one cause. There's a stack of them, and most are things you can change.

One thing up front: I couldn't find any authoritative figure for how fast iPhone SMB "should" be. The numbers below are what people reported in forums, on their own networks. Treat them as examples, not benchmarks.

What people report

The wide range is the point: the network and the server matter more than anything on the phone.

1. Wi-Fi is usually the bottleneck

Your NAS is probably wired. Your iPhone isn't. Wi-Fi speed depends on the standard, the band, the distance to the access point and how busy the air is, and SMB is sensitive to the extra latency Wi-Fi adds. Synology's slow transfer article starts with "Use a wired network instead of Wi-Fi".

2. Lots of small files

Every file has overhead: create it, write it, close it, set its attributes. With thousands of small files, that overhead dominates. In one Unraid forum test, around 95,000 files took 23 minutes to a normal user share and 6 minutes to a share on the cache drive.

If you're moving a big pile of small documents, zipping them first and copying one archive is usually much quicker. Photos and videos are bigger, so this matters less for a camera roll.

3. Server signing and encryption

Synology says plainly that "enabling server signing greatly reduces file transfer performance". Transport encryption costs CPU as well. On a Synology, set both Server signing and Transport encryption mode to Client defined, which is also the setting that avoids connection problems (see the Synology DSM 7.2 checklist).

You'll find Mac guides that tell you to turn off signing with /etc/nsmb.conf. That file only exists on macOS. There's no equivalent setting on an iPhone, and I couldn't find anything confirming whether iOS signs SMB traffic by default, so don't expect server-side signing changes to work miracles for the iPhone specifically.

4. The server's storage

A NAS writing to a busy hard drive array is slower than one writing to SSD. On Unraid (set up as in the Unraid and TrueNAS guide), user shares on the array are slower than shares on a pool; an Unraid moderator's advice is to make sure shares "have exclusive access enabled and are on a Pool, not the Array". If your NAS has an SSD cache or pool, point the iPhone's share at it. Real-time antivirus scanning on the NAS can also slow SMB writes.

5. The Files app itself

Some of the slowness people feel is the Files app's behaviour rather than raw speed. Users have observed it preparing a copy before you've even picked where it goes, the progress display tells you little, and folders with lots of items are slow to list and generate thumbnails for. A useful test is to copy the same files with another SMB app. If that's much quicker on the same network, you've learned the network isn't the whole story. The best SMB apps for iPhone lists the main options.

6. Remote access

Over a VPN, everything has to go through your home internet connection's upload speed, so it will be slower than at home, whatever the app. Synology's QuickConnect can't carry SMB at all. For remote access, a VPN such as Tailscale is the way to do it.

What not to expect

A quick plan

  1. Wire the NAS. Put the iPhone close to the router on 5 GHz or 6 GHz.
  2. Set signing and encryption to Client defined.
  3. Point the share at SSD or a cache pool if you have one.
  4. Zip big piles of small files.
  5. Compare with another app to see whether it's the network or the Files app.
  6. Do big transfers at home, not over a VPN.

Where SMBDrop fits

Disclosure: I make SMBDrop. I'm not going to tell you it's faster than the Files app or anything else. I haven't run a benchmark I'd publish, and the network and server matter more than the app anyway. What it does do is take the waiting off your hands: it sends original photos and videos (Live Photos keep both halves) in the background, and anything that fails goes into a retry queue instead of being lost. So a slow transfer can run while you do something else. It's £9.99 once, no subscription, no account. If you're copying videos, keeping them uncompressed matters more than shaving a few minutes.

If the transfer doesn't start at all, see the can't connect to server guide instead.

FAQ

How fast should iPhone SMB transfers be?

There's no official figure. Forum reports range from about 1 MB/s on an iPhone to 20 to 25 MB/s for an Android phone on good 5 GHz Wi-Fi, and they depend mostly on Wi-Fi and the server. A wired NAS and a close 5 GHz or 6 GHz connection give you the best chance.

Does the iPhone support SMB3 Multichannel?

Not as far as anyone documents. Synology lists Windows and macOS 11.3 or later as supported Multichannel clients, and iOS isn't on that list.

Does turning off SMB signing speed up iPhone transfers?

Synology says server signing greatly reduces transfer performance, so setting it to Client defined is sensible. Whether the iPhone signs SMB traffic by default isn't documented, so the gain on an iPhone is uncertain.

Why are lots of small files so slow over SMB?

Each file carries fixed overhead, so thousands of small files take far longer than one large file of the same total size. Zip them first and copy the archive.

More guides

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