This "weird group trick" makes MacBook Pro login, apps, and systems run slow slow slow ... but why?

Here's a "weird trick" that instantly makes my MacBook Pro / Yosemite system run slow slow slow ... create a circular group inclusion in the Users & Groups pane of System Preferences.

Answers Sought Why does this slowdown happen? Are other users affected? Is there a tool that automatically diagnoses and fixes it? How can it be avoided in the future?

Working Example

• create new groups "a" and "b"


• include "a" in "b"


• include "b" in "a"


• using the finder interface, recursively give "a" read/write privileges to any large directory tree (for me /usr/local is large enough to elicit a severe slowdown)

Voila! Now everything on my MacBook runs slow ... slow ... slow. Logins require up to one minute, and even simple typing sporadically elicits the "spinning pizza." Fortunately, eliminating the circular group inclusion instantly restores the system to its former speed.

Remark After creating this slowdown inadvertently, it took me two weeks to figure out, reproduce, and repair the cause of it. The search for a fix was slow and frustrating because none of the usual fixes of "slow Mac syndrome" were effective, and none of the usual diagnostics were revealing.

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System Software Overview:

System Version: OS X 10.10.2 (14C109)
Kernel Version: Darwin 14.1.0
Boot Volume: Macintosh HD
Boot Mode: Normal
Computer Name: [redacted]
User Name: [redacted] (Administration)
Secure Virtual Memory: Enabled

Hardware Overview:

Model Name: MacBook Pro
Model Identifier: MacBookPro11,3
Processor Name: Intel Core i7
Processor Speed: 2.5 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 6 MB
Memory: 16 GB
Boot ROM Version: MBP112.0138.B14

Note also that /usr/local tree is fairly large (as is typical of developers and LaTeX users) ...

$ find /usr/local -type f | wc -l
204713