Apple's Update CDN and Why It Can Be Slow
Apple delivers software updates through a combination of Akamai's CDN and Apple's own infrastructure, using domains such as apple.com, apple-cloudkit.com, mzstatic.com, and specific update domains like appldnld.apple.com and updates.cdn-apple.com. After a major iOS or macOS release, millions of devices request the same files simultaneously. Apple's CDN scales well, but ISP links to specific Akamai points of presence can become congested in the hours immediately following a release. Waiting 24–48 hours after a major release often results in noticeably faster downloads as demand spreads out.
iOS OTA Update Mechanics
An iOS over-the-air update downloads either a full IPSW (complete firmware image) or a delta package containing only the differences from your current version. Delta packages are much smaller — typically a few hundred MB rather than several GB — and are what most users receive when updating from a recent version. The device selects which package to request based on your current iOS version.
Before downloading, iOS verifies that you have adequate free storage — typically at least the size of the update package plus working space to expand and stage it. As a rough rule, you need at least 2× the update's stated size in free storage. If storage is tight, the update download will not start or will fail partway through. Go to Settings → General → iPhone Storage to see what is consuming space before attempting a large update.
iOS Update Prerequisites
Three conditions must be met for an iOS or iPadOS OTA update to download and install reliably: the device must be connected to power (or have sufficient battery charge — typically above 50% for download, with power required for install), it must be on Wi-Fi rather than cellular, and it must have adequate free storage. Background App Refresh and certain power-saving restrictions can also pause or throttle update downloads. Turn off Low Power Mode in Settings → Battery before starting a large update.
Common Apple Update Slowdowns
| Symptom | Likely Cause | Best Fix |
|---|---|---|
| Update crawls on one iPhone | Storage shortage, Low Power Mode, or stuck partial file | Free space, charge device, delete partial update |
| All Apple devices slow simultaneously | Apple CDN congestion or ISP peering to Akamai | Try again later, change DNS, disable VPN |
| Download stops when screen locks | Power saving throttling background network activity | Plug in, disable Low Power Mode, keep screen on |
| Browser fast, update slow | Apple CDN server load or specific content route | Wait or test from a different network |
| Repeated failures at same point | Corrupted partial download file | Delete the downloaded update and re-request |
macOS Update via Command Line
On macOS, Software Update can be driven from Terminal, which shows more verbose progress than the System Settings UI and is useful for diagnosing whether the update is downloading, verifying, or stuck:
softwareupdate --list
softwareupdate -i -a --verboseThe --verbose flag prints download progress, verification steps, and any error messages to the terminal. If the GUI shows no progress but the terminal shows bytes downloading, the update is working — the GUI percentage just is not updating in real time.
For deeper diagnosis on macOS, check /var/log/install.log and /var/log/system.log for update-related entries. Filter for "SoftwareUpdate" or "osinstallersetupd" to find relevant lines. These logs often reveal whether a failure is a network error, a verification error, or a disk space issue.
Apple Silicon vs Intel Download Behavior
Apple Silicon Macs (M1 and later) and Intel Macs download different IPSW firmware files for the same macOS version. The IPSW for Apple Silicon is typically larger because it includes additional firmware components for the Secure Enclave, T2-equivalent chips, and the unified memory architecture. If you are comparing download times between an M-series Mac and an older Intel Mac on the same network, the M-series download will take longer even if both are downloading at the same speed, simply because the file is larger.
Fix 1: Delete a Stuck iOS Update and Re-download
If an iOS update has been partially downloaded but fails repeatedly, it is often because the partial file is corrupted or the download session has expired. Remove it from device storage: go to Settings → General → iPhone Storage, scroll down to find the update listed as a separate entry (it appears as "iOS [version] Software Update"), tap it, and delete it. Restart the device, then go to Settings → General → Software Update to request a fresh download. This resolves the majority of cases where an update keeps failing at the same progress point.
Fix 2: Use Finder or iTunes for a Stubborn iOS Device
Updating through a computer bypasses the OTA download entirely. Connect the iPhone or iPad via USB, open Finder on macOS (Catalina or later) or the Apple Devices app on Windows, select the device, and click Check for Update. The update IPSW is downloaded to the computer rather than the device, avoiding device storage constraints and on-device Wi-Fi limitations. This is especially useful when the device has very little free space or a weak Wi-Fi connection that keeps interrupting the download.
Fix 3: Turn Off VPN and Check DNS
VPNs change the network path Apple's update servers use to reach your device. If the VPN exits through a distant country or a congested server, update downloads routed through it will be slow. Disable the VPN before starting the update. If you use iCloud Private Relay (available on paid iCloud plans), try disabling it temporarily in Settings → [your name] → iCloud → Private Relay — it occasionally affects the CDN endpoint selected for software updates. Changing your DNS to 1.1.1.1 or 8.8.8.8 can also route you to a closer Akamai node for Apple's CDN.
Frequently Asked Questions
Why is my iPhone update downloading so slowly?
The most common causes are weak Wi-Fi signal, Low Power Mode throttling background downloads, insufficient free storage preventing the full download, Apple CDN congestion shortly after a major release, or a VPN routing the download through a slow exit node.
Does Apple Private Relay slow software updates?
Private Relay routes web traffic through two separate internet relays for privacy, which can occasionally affect which Apple CDN node your device reaches for update downloads. Temporarily disabling it is a useful test if your update is unusually slow and your connection is otherwise fast.
Should I update iPhone through a computer if Wi-Fi is slow?
Yes. Updating via Finder on macOS or the Apple Devices app on Windows downloads the IPSW to the computer rather than the phone, bypassing device storage limits and on-device Wi-Fi instability. It is the most reliable method when OTA updates keep failing.