A
When you see a beachball cursor or the slowness is especially bad, note the exact time: hour, minute, second.
These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.
Launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad and start typing the name.
The title of the Console window should be All Messages. If it isn't, select
SYSTEM LOG QUERIES ▹ All Messages
from the log list on the left. If you don't see that list, select
View ▹ Show Log List
from the menu bar at the top of the screen.
Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.
Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.
Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.
The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.
Please don't indiscriminately dump thousands of lines from the log into this discussion.
Please don't post screenshots of log messages—post the text.
Some private information, such as your name, may appear in the log. Anonymize before posting.
When you post the log extract, you might see an error message on the web page: "You have included content in your post that is not permitted," or "The message contains invalid characters." That's a bug in the forum software. Please post the text on Pastebin, then post a link here to the page you created.
B
One of the myths that circulate on this site is the myth that you should maintain a certain percentage, such as 10% or 15%, of free space on the startup volume. That myth is based on nothing except the fact that people keep repeating it.
According to Apple documentation, you need at least 9 GB of available space on the startup volume (as shown in the Finder Info window) for normal operation. You also need enough space left over to allow for growth of the data. There is little or no performance advantage to having more available space than the minimum Apple recommends. Available storage space that you'll never use is wasted space.
[Further]
If my earlier comment needs to be clarified—which I don't think it does—there is no basis in evidence or common sense for the "10-15% free space" myth. The only basis for it is the fact that people keep repeating it. The support article I cited is the best information available on the subject.
The amount of free space you need is not proportional to the size of the storage device. It depends entirely on your usage pattern. For example, if you have a 256 GB SSD with 128 GB of free space, that's 50% free. Suppose you're adding data at the rate of 4 GB a day. Then you will run out of space in a month. 50% free space is not enough for you. On the other hand, if you have only 12 GB free, which is about 5%, and you never add any net data at all, then the drive will never fill up. 4% free space is adequate. Regardless of how much space is free, if the drive mysteriously fills up without your having added any data, then something is wrong, and it would be just as wrong if you had more free space.
The performance of a hard drive in certain operations degrades gradually, and most often unnoticeably, as it fills up. The performance of an SSD has a more complicated dependence on free space. In neither case does performance fall off a cliff when the 10% or 15% threshold is crossed.
You will ask in vain for documentation or published engineering data to support the myth. The closest thing to it was an Apple document from about 20 years ago that someone showed me once. It actually did recommend keeping 10% of the startup volume free. That may be how the myth got started. But the document applied to the old pre-Next Mac OS, and it was written when the typical startup volume was about a thousand times smaller than it is now. It has no bearing on the modern world, and arguably it was inaccurate even at the time.