Mac Slow While a Backup Is Running? Here's Why

Backup software has a thankless job. It needs to read every file that’s changed since the last backup, write that data to a separate disk (or cloud), verify integrity, and ideally do it all without you noticing. On a Mac, the default backup tool — Time Machine — gets credit for being mostly invisible. Until it isn’t.

When backups slow your Mac to a crawl, the cause is usually predictable: a backup is initial (so it’s reading everything), the destination is slow, or some specific file pattern is interacting badly with the backup engine.

What Time Machine Does in the Background

Time Machine runs as a daemon. The relevant processes:

• backupd — the main backup daemon
• mdworker_shared — Spotlight, indexing files Time Machine touches
• fseventsd — filesystem event tracking, used to detect what’s changed
• APFS snapshot daemons — for the local “Snapshots” feature

When Time Machine kicks off:

1. Compare current filesystem state to the last backup
2. Identify all changes (additions, modifications, deletions)
3. Walk through each changed file, reading and copying
4. Verify the copy matches the source
5. Write metadata indicating the backup is complete

The first backup of a Mac is the heaviest because everything is “changed.” Subsequent backups only handle changes — usually a fraction of total disk size.

Free up RAM in one clickSweep frees inactive memory and pauses runaway processes. Get Sweep free →

The Initial Backup Problem

If you’ve just set up Time Machine, the initial backup can take 4-12 hours depending on:

• How much data you have (500GB takes longer than 100GB)
• Destination disk speed (USB 2.0 is glacial, USB-C SSD is fast)
• Connection type (wired beats Wi-Fi for Time Capsule-style network backups)
• Whether you’re on battery (macOS throttles backups on battery)

During this time, your Mac will:

• Run slower than normal
• Have noticeable disk activity continuously
• Possibly have fan noise from sustained CPU
• Drain battery faster

Plug in to power. Use a fast destination disk. Don’t try to do this on a 2TB Mac with a USB 2.0 spinning disk — it’ll never finish in any reasonable time.

After the first backup, ongoing backups (every hour by default in older macOS, less frequent now) handle only changes and finish in minutes.

Why “Preparing Backup” Takes Forever

A common Time Machine pain point: it shows “Preparing backup…” for 30+ minutes before any data actually transfers.

What’s happening: Time Machine is comparing your current state to the last backup. For Macs with millions of files (especially developers with node_modules, scientists with large datasets), this comparison itself takes a long time.

The “Preparing” phase involves:

1. Reading the file system event log
2. Cross-referencing it with the backup destination’s catalog
3. Building a list of files to back up
4. Calculating checksums for changed files

If your Mac has experienced a system crash, sleep issue, or file system corruption since the last backup, Time Machine may need to do a “deep traverse” — scanning every file directly instead of using the event log. This is much slower.

To check what Time Machine is doing right now:

log stream --predicate 'subsystem == "com.apple.TimeMachine"' --info

You’ll see real-time messages about what backupd is processing.

Excluding the Right Folders

Time Machine backs up everything by default. For most files, this is good. For some, it’s wasteful or actively harmful.

Worth excluding:

1. Cache folders — ~/Library/Caches/, /private/var/folders/. They regenerate, no point backing up
2. Virtual machine disk images — backing up a 50GB VM that changes constantly is wasteful
3. Docker images — same logic, plus they’re available from registries
4. node_modules — recreated by npm install
5. DerivedData — Xcode regenerates
6. Downloaded installer DMGs — usually re-downloadable
7. Previous Libraries.localized in Music — backups of backups
8. iCloud Drive folder — already backed up to iCloud, redundant unless you want local
9. Downloads folder — depends on workflow, but often most contents are downloads you’d re-download

To configure: System Settings > General > Time Machine > Options > exclude folders.

Excluding these dramatically reduces backup time, destination disk usage, and CPU spent on the verification step.

Local Snapshots: The APFS Bonus (and Cost)

Modern Time Machine uses APFS local snapshots — point-in-time copies stored on your Mac itself, alongside the regular backup destination. They let you “go back” even when your backup drive is disconnected.

Local snapshots use space proportional to how much data has changed. For a typical Mac, 5-15GB. They’re managed automatically — older snapshots are pruned when space gets tight.

When local snapshots cause slowdowns:

• Your boot drive is nearly full and snapshot management is constantly running
• A specific snapshot is corrupted and macOS keeps trying to verify it

To see current snapshots:

tmutil listlocalsnapshots /

To delete all local snapshots (frees space, doesn’t affect external backups):

sudo tmutil deletelocalsnapshots <date>

Or for all of them:

sudo tmutil thinlocalsnapshots / 999999999999 4

Sweep also surfaces local snapshots in its scan and shows the space they occupy.

Skip the manual huntSweep finds the buildup slowing your Mac and clears it in seconds. Download Sweep free →

Network Backup: Time Capsule, NAS, and Why They’re Slow

Backing up to a network destination is convenient but inherently slower than direct-attached storage. Common patterns and their performance:

1. Time Capsule (Apple’s, discontinued but still used) — modest speeds, often the bottleneck
2. AirPort Extreme with a USB drive — slower still
3. NAS like Synology or QNAP with Time Machine support — speed depends entirely on the NAS hardware and your network
4. Direct Wi-Fi link to a Mac running Time Machine server — variable, often not great

For network backups, throughput is often the bottleneck. A backup that takes 20 minutes on USB-C SSD might take 4 hours over Wi-Fi.

If network backup is slow:

• Use 5GHz or 6GHz Wi-Fi rather than 2.4GHz
• Connect to your network via Ethernet if possible (even a cheap USB-C to Ethernet adapter helps)
• Make sure your NAS has SSD caching or is otherwise reasonably fast
• Consider a hybrid: external SSD for daily backups, NAS for weekly archives

Third-Party Backup Tools and Their Weight

Many Mac users supplement Time Machine with cloud backup (Backblaze, Carbonite, Arq) or local clones (Carbon Copy Cloner, SuperDuper).

Each adds its own background process:

• Backblaze runs bzbmenu and bzfilelist continuously
• Carbonite has its own daemon
• Carbon Copy Cloner schedules tasks
• Arq runs scheduled snapshots

These can compound with Time Machine if not coordinated. Two backup tools both reading every file change at once is heavier than either one alone.

Where possible, stagger schedules. Time Machine for hourly local + Backblaze for continuous cloud is a common setup that works well. Two cloud backup tools running simultaneously is usually overkill.

There’s a faster waySweep diagnoses what’s eating your performance and clears it in seconds. Try Sweep free →

A Diagnostic for Backup-Related Slowness

When your Mac drags during a backup:

1. Check Activity Monitor. Look for backupd, mdworker_shared, fseventsd
2. Check the Time Machine status — menu bar icon
3. Check destination disk speed — if it’s USB 2.0 spinning, that’s the issue
4. Check exclusions — are you backing up things you don’t need?
5. Cancel the backup to verify it’s the cause: tmutil stopbackup in Terminal
6. Check local snapshots — too many means storage management overhead
7. Verify boot drive isn’t nearly full — backups need working space
8. Pause Time Machine during heavy work: tmutil disable (re-enable with tmutil enable)

Settings Worth Changing

In macOS:

• System Settings > General > Time Machine > Options > Back up while on battery: OFF (default in modern macOS)
• System Settings > General > Time Machine > Options > Notifications: ON so you know if backups are failing
• Battery > Power Adapter > Prevent automatic sleeping when display is off: ON if you backup overnight

In your daily workflow:

• Plug in before leaving if you want overnight backups to run
• Wake your Mac while plugged in to let backups catch up
• Check the Time Machine menu weekly to verify backups are succeeding

When Time Machine Is the Wrong Tool

For some Mac users, Time Machine isn’t a great fit:

• Many small files changing constantly — Time Machine’s per-file approach is slow
• Very large files changing slowly — incremental rates are inefficient
• Highly sensitive data — Time Machine encrypts but you may want stronger options
• Need for versioned cloud storage — Backblaze, Arq, or rsync.net are better

For most Macs, Time Machine plus a cloud backup is the right answer. For specialty workflows, consider alternatives.

Long-Term Habits

To keep backups from quietly slowing your Mac:

• Verify backups complete — at least weekly check the menu bar status
• Maintain exclusions — as your workflow changes, update what’s excluded
• Replace backup drives — they fail. SSDs degrade after years. Have at least one backup of your backup
• Don’t run two heavy backups simultaneously

Sweep keeps your boot drive clean — caches cleared, language files trimmed, large old files surfaced. A Mac with healthy free space backs up faster, has fewer snapshot issues, and recovers from problems more cleanly. Combined with sensible Time Machine settings, backups stay invisible the way they’re supposed to.