M1 iMac experiencing lag due to low storage space

Your iMac has about 426 GB of 494 GB of storage in use, with approximately 32 GB reported as available by Disk Utility. This is worth investigating as part of the performance problem, but it does not establish that limited storage is causing the lag.

First, check  > System Settings > General > Storage. 

This provides macOS's breakdown of how the internal drive is being used. 

Because there are two user accounts, storage used by files belonging to either account should be considered.

The 140 GB shown in your iCloud folder does not necessarily mean that all 140 GB is stored locally. 

iCloud Drive can keep files in iCloud while macOS manages which files are also retained on the Mac. 

Check Optimize Mac Storage under  > System Settings > your name > iCloud > Drive. 

Apple explains this here: Set up iCloud Drive.

If you want to free space, consider moving large personal files, particularly photos, videos, music, and other files you control, to an external SSD. 

This can free internal storage without attempting to remove files that macOS manages as System Data. 

Make sure important files are backed up before moving or deleting them.

Using Time Machine Backup Utility  or some other from of backing up your personal files 

After freeing some storage, use the Mac normally and observe whether the lag changes. 

If it does not, investigate other possible causes rather than assuming that storage is responsible.

Activity Monitor can be used while the Mac is lagging to examine CPU usage, memory pressure, and disk activity.

Apple provides information here: View CPU activity in Activity Monitor on Mac.

Apple's guidance for managing Mac storage is available here: Manage storage space on Mac.

You should really try to maintain somewhere between 80-100GB of free space for smooth trouble free operation.

You can use iCloud and Optimization to gain storage space, if you are willing to subscribe and pay for that space.

Free up storage space on Mac - Apple Support

To gain large chunks of free space, you can move your Photos, Music or Movie Libraries to an external hard drive.

Move your Photos library to save space on your Mac - Apple Support

Change where your music files are stored on Mac - Apple Support

Move Logic Pro for Mac and MainStage content - Apple Support

Move your iMovie for Mac library - Apple Support

Move a Final Cut Pro library - Apple Support

After you've read Owl-53, den.thed and Alan's comments and recommendations you might do what I've done:

1 - get a 1TB external SSD drive and format it APFS with GUID partition.

2 - copy your Picturtes and Music folders to the external Drive

3 - verify that you Photos and Music libraries work as expected from the external drive before deleting the contents of those folders on your internal boot drive.

4 - delete the "contents" of the Pictures and Music folders, not the folder themselves.

5 - you can add alias files for the Photos library and Music library in the folders if you'd like.

If you follow the above you should be able to free up the 80-100 GB of space to facilitate optimal system and application memory.

I prefer drives from OWC (MacSales.com). They are a bit more expensive than others but have excelent reliably and a very good warranty. Just some food for thought.

Good morning!

Did you recently update to macOS 27? For me, it is using more disk space for some things than did Tahoe.

Using System Settings > About > Storage,I found that a second restart about a day after installing reduced the size of the "System Data" category from 70GB to 54GB. Not a big change but, in your case, one that may help incrementally.

Your observation:

Both Tahoe and GG are very laggy. I assume this is due to lack of free space.

is exactly on-target. Virtual Memory (VM)—the writing of data to storage instead of RAM—runs all the time. Although storage in M-series Macs is fast, it is not as fast as RAM. Overly-full storage runs slower than when you have 20% or more free space. That slows VM performance. Having 16GB RAM is a big step toward mitigating the VM issue, but so does having free space. A crowded drive is a slow drive.

When you replace this iMac, "think big" on up-ordering storage. I see far more issues here with too little storage than too little RAM. With the recent bloating of a macOS 27 install, I now recommend a minimum of 1TB storage as "future-proofing." Having two users alone justifies the added expense.

Good additional points / tips 🦉-53