Why a NotchStar Disk scan can be slow
Disk scans are slow when you have a lot of files, because they read real data from the disk. No timings have been measured, since they depend on how many files you have and what kind of drive holds them. This page says what each scan does so you can see where the time goes.
What each scan does
| Section | Work | Why it takes time |
|---|---|---|
| Overview | Adds up the size of every item in caches, logs, Xcode data and Downloads | Caches can hold very many small files |
| Space | Sizes every folder in your home folder, four at a time | The Library folder is often the largest thing you own |
| Find | Walks your home folder and matches names | Every file and folder is looked at |
| Duplicates | Groups by size, hashes the ends, then hashes the whole file | Full hashing reads each candidate completely |
| Apps | Measures each app and its leftovers | Large apps are big folders |
| Compress | Re encodes each chosen file | Video encoding is the heaviest work in the tab |
Things that make it slower
- External or network drives, or folders synced by a cloud service.
- A first scan of Documents, Desktop or Downloads, when macOS asks for access.
- Very large media libraries in Movies and Pictures.
- Other heavy work on the Mac at the same time.
Why it is built this way
Most scans run as low priority background tasks, and they start only when you press a button. Nothing runs in the background. Results appear as they are measured in Space, so you can start reading before it finishes.
What to do
Scan one section at a time, and begin with Space to see where the weight is. Duplicates is narrower: only files of 1 MB or more in six folders. See how duplicates are found and how to find the biggest folders. The Disk feature page lists everything the tab does.
