It all started after a macOS Big Sur upgrade. One moment, everything was smooth. The next, my fans kicked into overdrive, my apps stuttered, and my CPU was maxed out. The culprit? An app I trusted—Little Snitch. Here’s how I dug into the issue, tracked down the cause, and fixed it with a bit of rule cleanup magic.
TL;DR
After updating to macOS Big Sur, Little Snitch caused high CPU usage that slowed the system. The problem came from old or overly complex rules that didn’t play well with the new OS. A full audit and rebuild of those rules made everything run like new again. If your Mac feels hot and laggy post-upgrade, check your network filter settings!
What Is Little Snitch Anyway?
Little Snitch is like a bouncer for your Mac. It watches incoming and outgoing network connections. If an app wants to send data online, Little Snitch asks for your permission. Super helpful! It gives you control and visibility.
But sometimes, when the system changes drastically (like after a major OS update), it gets confused. And that’s exactly what happened in this case.
After Big Sur: Things Got Rough
Once I upgraded to Big Sur, I noticed something odd. My MacBook’s fans were running all the time. Certain apps lagged or froze. Activity Monitor showed a shocking discovery.
Little Snitch Network Extension was eating up 80-100% of my CPU. Constantly.
I ran a few tests. I rebooted. I reinstalled Little Snitch. I blamed the weather. But no change.
Clues Point to the Rule Set
After reading through forums and user reports, a pattern popped up. Many users had large, outdated, or messy rule sets. These were rules built up over months or years of usage. And now, Big Sur didn’t seem to play nice with them.
It was time for a network rule detox.
Step 1: Export the Old Rules
Before touching anything, I backed up my current rules. Here’s how:
- Open Little Snitch Configuration
- Go to “Rules” menu
- Select “Export Rules…” and save the file
This way, if things broke later, I could quickly revert.
Step 2: Audit the Chaos
I started combing through my rules. I had nearly 500+ custom entries. Some were duplicates. Others were outdated. A few referred to apps I hadn’t used since 2017.
Here’s what I looked out for:
- Rules for apps I don’t use anymore
- Redundant entries (same domain or IP multiple times)
- Generic “allow all connections” stuff tied to suspicious or wide-scope ports
Every time Little Snitch had to check these rules during a connection, it was doing a LOT of work. No wonder the CPU was unhappy.
Step 3: Start Fresh
Feeling bold, I decided to do a full rebuild. Fresh start. Like Marie Kondo, but for network permissions.
I deleted all my existing rules. Yup. Poof. Gone.
Then I rebuilt them only as needed.
Here’s how I did it smoothly:
- Disconnect from the internet
- Clear all existing rules
- Re-enable monitoring in Little Snitch
- Reconnect to the internet
- Go about normal app usage. Only allow when absolutely needed.
This way, I created a lean, clean rule set as I worked. No more bloated backlog of random stuff from 2016.
Surprise Bonus: My Mac Was Faster
After the rebuild, the difference was night and day.
- Fans became silent
- CPU load dropped by 30-50%
- Apps responded quicker
- Battery life noticeably improved
Turns out, that old rule junk was weighing down the whole system. Maybe it wasn’t ALL Little Snitch’s fault. Part of the issue was my laziness in managing those rules over the years.
Tweaks That Helped Even More
After the cleanup, I also changed a few settings in Little Snitch to reduce background load:
- Reduced automatic DNS lookups: Less traffic = Less work
- Turned off silent mode logging: No need to log denied attempts I already told it to ignore
- Set some trusted apps to “Always Allow”: Like Safari or UpdateDaemon
Each of these shaved a bit more load off the CPU. Like giving your Mac a bubble bath and a nap.
Why This Happens After Big Sur
macOS Big Sur introduced major system changes. New frameworks, hardened extensions, different permission models. Little Snitch had to adapt too.
And when an app like this checks EVERY network packet, it really suffers if it’s trying to match that against hundreds of rules built using old APIs or based on outdated paths.
So yes, sometimes you need to rewind and restructure.
Lessons Learned
This whole adventure taught me a few solid tips:
- Keep your firewall and network rules clean
- After major OS upgrades, always check compatibility settings
- Auditing is not just for accountants
- Your fans aren’t lying—when they scream, they mean it
Conclusion
Little Snitch is still one of my favorite Mac tools. But it needs care. Especially after a system change.
If you’re having weird slowdowns after upgrading macOS, don’t forget to check your network tools. Especially ones that sit low-level in the OS like Little Snitch.
A good rule audit and fresh rebuild might be all you need to cool things down and get your Mac humming again.
Happy snitching!