Why File Copies Are Slow on Mac (and How to Speed Them Up)

You drag a folder onto an external drive expecting it to take a minute. The progress bar appears. Says “About 47 minutes remaining.” Crawls to 12%. Then jumps to 73%. Then back to “calculating time remaining.” By the time it finishes you’ve checked your phone, gotten coffee, and forgotten what you were doing.

File copy speed on a Mac depends on more variables than most people realize: source disk type, destination disk type, file size distribution, the file system on each end, encryption status, indexing activity, and whether Time Machine decides to snapshot mid-transfer. Here’s how to figure out which one’s slowing you down.

What determines copy speed

The bottleneck is whichever step is slowest in this chain:

1. Read speed of source disk
2. CPU time for encryption/decryption (FileVault) and any compression
3. Memory buffering between read and write
4. Write speed of destination disk
5. Filesystem overhead (HFS+ vs APFS vs exFAT vs FAT32)
6. Background indexing on either end (Spotlight, Finder thumbnails)

For an internal SSD-to-SSD copy on a modern Mac, you should see 1+ GB/s easily. For an external SSD over USB-C, expect 400-1000 MB/s depending on the cable and enclosure. For a USB-A external HDD, you’re stuck at 100-130 MB/s no matter what.

If you’re seeing dramatically less than expected, something’s wrong. Probably one of the things below.

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Fix 1: Check your cable and port

This sounds dumb but it’s the number one cause of “my drive is slow.” USB-C cables are not all created equal:

• Some USB-C cables are USB 2.0 only (480 Mbps = 60 MB/s real-world). Plug a Thunderbolt SSD into one of these and copies crawl.
• Cables that ship with phone chargers are often USB 2.0
• Even some “thunderbolt” cables only support charging, not data
• USB-A on older Macs or hubs is USB 2.0 unless explicitly USB 3

Test by plugging the same drive into different ports with different cables. If speed varies, you’ve found your problem. Mark the slow cable and throw it out — it’ll keep biting you.

Fix 2: Check the destination filesystem

Drag a 5 GB file to your external. While it copies, run this in Terminal:

diskutil list

Find your external drive. Look at the Type column.

• APFS — macOS native, fastest for Mac-to-Mac
• Mac OS Extended (HFS+) — older but still fast
• ExFAT — cross-platform, but noticeably slower especially for many small files
• MS-DOS (FAT32) — slowest, also can’t hold files over 4 GB

If your drive is exFAT and you only use it on a Mac, reformat to APFS. The speed difference for many small files is enormous — sometimes 5-10x.

To reformat:

1. Back up everything on the drive first
2. Open Disk Utility
3. Select the drive (the physical drive, not just the partition)
4. Click Erase, choose APFS, give it a name
5. Click Erase

Fix 3: The “many small files” problem

A single 10 GB file copies way faster than 100,000 files totaling 10 GB. Each file has overhead — directory entries, permissions, extended attributes, resource forks (yes, still). For tens of thousands of small files, that overhead dominates total copy time.

Workarounds:

• Compress before copying: zip the folder first. Single file = much faster transfer. Then unzip on the destination.
• Use ditto or rsync in Terminal — they handle small files more efficiently than Finder
• Skip Finder’s progress UI for huge transfers — it adds overhead

For the rsync approach:

rsync -av --progress /source/folder/ /destination/folder/

The -a preserves permissions and times. Significantly faster than Finder for many-files transfers.

Fix 4: Pause Time Machine

If a Time Machine backup kicks off during your copy, both processes are fighting for disk I/O. Time Machine snapshots happen frequently — every hour by default — and they can absolutely tank a copy in progress.

Turn it off temporarily:

1. System Settings → General → Time Machine
2. Click “Options” (or “Back Up Automatically” toggle)
3. Disable while you do the big copy
4. Re-enable when done

You can also right-click the Time Machine menu bar icon and skip the in-progress backup if one’s running.

Fix 5: Turn off Spotlight indexing for the destination

When you copy files to a new external drive, Spotlight starts indexing every file as it arrives. This doubles the disk I/O — write the file, then read it back to index — and slows the copy substantially.

To exclude a drive from indexing:

1. System Settings → Spotlight → Search Privacy
2. Click + and add the external drive
3. The drive won’t be searchable from Spotlight, but copies will be much faster

You can remove it from the privacy list later to enable indexing.

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Fix 6: Check what’s eating disk I/O

Open Activity Monitor. Click the Disk tab. Sort by “Bytes Written” and “Bytes Read.”

Common offenders during slow copies:

• mds, mds_stores — Spotlight, see fix 5
• backupd — Time Machine, see fix 4
• bird — iCloud Drive sync
• photoanalysisd — Photos library analysis
• MailServiceAgent — Mail importing/indexing
• Notes — note sync
• Antivirus apps (anything from Norton, McAfee, Sophos, etc.) — these can scan every file as it’s copied

If a third-party security app is high on the disk list during a copy, temporarily disabling its real-time scanning might be the fix. Re-enable when done.

Fix 7: Free up disk space on both ends

Both the source and destination drives need free space to operate at full speed. APFS especially — once a drive is over 90% full, write speeds drop noticeably because the filesystem has less room to work with.

Check both:

• Internal drive: Apple menu → About This Mac → More Info → Storage
• External: select in Finder, Get Info, look at Available

If either is under 15% free, that’s your slowdown right there.

Fix 8: Check for thermal throttling

Sustained file copies push CPU and SSD controllers hard. On Macs with limited cooling — MacBook Air, base Mac mini Intel — they thermally throttle after a few minutes of sustained work.

Symptoms:

• Copy starts fast, slows after 2-3 minutes
• Mac feels hot to the touch
• Fans (if present) are audibly maxed out

Solutions:

• Don’t put your MacBook on a soft surface during big copies (blanket, pillow, lap)
• Use an external fan or cooling pad
• Break the copy into smaller chunks with breaks
• Plug into power if on battery — battery-only mode often throttles harder

Fix 9: Disable FileVault temporarily? (Probably don’t.)

FileVault adds CPU overhead for encryption. On Apple Silicon Macs the impact is essentially zero — it’s hardware-accelerated. On Intel Macs it can be measurable, especially older ones.

That said: do not disable FileVault to speed up copies. The data security benefit massively outweighs the speed difference, and disabling/re-enabling FileVault takes hours. If FileVault is your bottleneck on an old Intel Mac, the answer is a new Mac, not insecure data.

When it’s actually the drive itself

Some external drives are simply slow:

• USB flash drives are almost always slow for sustained writes (the controllers run out of cache)
• Old portable HDDs were never fast — 80-100 MB/s peak
• Cheap “1TB SSD” drives from random brands often have poor sustained write speed
• Old SD cards — even high-class ones — can sustain only 30-60 MB/s

A USB-3 SSD from a reputable brand (Samsung, SanDisk, OWC) should hit 400+ MB/s sustained on any modern Mac. If yours doesn’t, the drive is the bottleneck.

The cleanup angle

A Mac that’s full and tired copies files slowly. The internal SSD does double duty as cache for external transfers — when it’s bogged down with caches, logs, and forgotten downloads, even external-to-external transfers via your Mac suffer.

Standard cleanup paths to check:

• ~/Library/Caches/ — cache files for every app you’ve ever run
• ~/Library/Logs/ — old logs
• ~/Downloads/ — the obvious one
• Old .dmg files left after installing apps
• Old Time Machine local snapshots (these can take hundreds of GB)

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A practical workflow for big transfers

If you regularly move large amounts of data:

1. Format external drives as APFS (Mac-only) or exFAT (cross-platform)
2. Add external drives to Spotlight Privacy if you don’t search them
3. Pause Time Machine before big copies
4. Use rsync from Terminal for many-small-file transfers
5. Use Finder for single large files (it’s fine)
6. Check cable + port if speed is unexpectedly low

That’s the recipe. Most “slow copy” complaints get fixed by one of these six adjustments.