One of the biggest penalties in using disk images has been their performance, particularly when they’re encrypted. Although no longer offered in Disk Utility, UDSP sparse images encrypted using 256-bit AES typically read and write as slow as 500/100 MB/s when mounted from an SSD delivering 4.7/4.9 GB/s. In contrast, UDSB sparse bundles can achieve close to that native speed.

macOS Sequoia brought a new type of disk image, Apple Sparse Image Format or ASIF, intended to deliver the high performance of sparse bundles, with their efficient use of storage space, in a single file that can be hosted on file systems beyond APSF. As this is now well over 18 months old, this article considers whether it has achieved those goals, and should become the preferred type of disk image.

Methods

Each test image was created using Disk Utility 22.7 (2510) in macOS 26.3.1 (a) running on a Mac mini M4 Pro, on its internal SSD of 2 TB. Performance measurements were made using the ‘gold standard’ method in my free Stibium on disk images of 100 GB nominal size. This writes and reads a total of 53 GB in 160 files ranging in size between 2 MB and 2 GB. As performance is likely to change with use of the disk image, the following sequence of events was used:

• Create disk image, which is mounted automatically, so unmount.
• Mount disk image, measure write speed, then unmount.
• Mount disk image, measure read speed, then delete the last 8 of 10 sets of test files, and unmount.
• Mount disk image, measure write speed, then unmount.
• Mount disk image, measure read speed, then delete all test files, and unmount.
• Mount disk image, measure write speed, then unmount.
• Mount disk image, measure read speed, and unmount.

These provide three pairs of read/write measurements:

1. for an empty, unused 100 GB disk image;
2. for a 100 GB disk image containing over 10 GB of existing files in addition to 53 GB of test files;
3. for an empty 100 GB disk image that had previously contained about 66 GB of test files that had been deleted.

Disk image sizes were also measured when unmounted, using Precize or the Finder’s Get Info (for sparse bundles).

The three types of disk image tested were RAW (UDRW), UDSB (sparse bundle) and ASIF (sparse image). Each was tested fully when unencrypted, and test 1 was performed on an image encrypted using 256-bit AES.

Results

The best and most consistent performance was achieved by UDSB sparse bundles, as expected. Their read speeds were 6.13, 6.12 and 6.19 GB/s, and write 7.62, 8.03 and 7.79 GB/s for the three separate measurements, and 5.09/5.23 GB/s read/write when encrypted. When first created, the sparse bundle only occupied 32 MB on disk, but by the end had grown to 3.99 GB even though empty.

The RAW disk image, formerly known as UDRW, also largely performed as expected. Read speeds were 6.09, 6.10 and 6.08 GB/s, and write 10.11, 9.86 and 10.11 GB/s. Initially it only required 5.78 MB on disk, rising to 621 MB at the end. However, its performance when encrypted was disappointing, at 2.84/1.58 GB/s read/write.

ASIF disk images were good, but also ran into problems when encrypted. Unencrypted read speeds were 5.99, 5.88 and 5.85 GB/s, and write 9.55, 8.93 and 9.64 GB/s. When encrypted, those fell to 2.82/1.72 GB/s read/write, no better than the RAW disk image. The image file size started at 26.8 MB on disk when empty and unused, and returned to 954 MB when empty at the end.

To confirm that ASIF performance when encrypted wasn’t an anomaly, I repeated that pair of tests on a MacBook Pro M3 Pro running 26.3.1 (a), and obtained similar results at 2.63/1.52 GB/s read/write, using a 10 GB ASIF image with one-tenth of the tests, giving 3.32/1.65 GB/s, and using Blackmagic, which gave 2.92/1.15 GB/s read/write. Although there is variation, they appear remarkably similar.

Test 2 results are summarised in the table above, for ease of comparison, and with the earlier results from macOS 26.0 below.

What has gone wrong with encrypted writes?

Although most of the test results in macOS 26.3.1 are very similar to those from 26.0, performance when using 256-bit AES encryption has fallen for all three disk image types, and most significantly in write performance for RAW and ASIF images, which have reduced from 4.3 to 1.58 GB/s (RAW) and from 3.9 to 1.72 GB/s (ASIF). The magnitude of those reductions is sufficient to have obvious impact on their use. Compared to native write performance using FileVault of 7.66 GB/s, those two types of disk image are pedestrian in the extreme, turning that blisteringly fast SSD into the equivalent of 20 Gbps over USB 3.2 Gen 2×2.

It’s possible that this dramatic reduction in encryption performance may have resulted from a change to address a vulnerability, but I’ve been unable to identify an entry in Apple’s security release notes that might correspond to such an event. I will repeat these tests once the update to macOS 26.4 has been released, in the hope it might be reversed.

Which disk image type?

When their folder-based structure is acceptable, UDSB sparse images remain the disk image type of choice, for their consistent high performance even when encrypted.

There is little to choose between RAW and ASIF disk images when a single file solution is required. ASIF images are portable to other file systems that can’t support APFS native sparse files, although curiously they too are flagged in APFS as being sparse files. As their sparseness isn’t dependent on APFS trimming habits, they are now an alternative that can be used on network storage and NAS. However, those able to use sparse bundles should continue to do so, particularly if using encryption.