It happens to every Mac user sooner or later. The virtual gears inside your computer begin to act as though they're running in a vat of tapioca pudding. No matter what you try to do, your Mac moves at a pace that a snail could run circles around. But before carting your Mac off to an Apple Genius Bar, try these fixes.
Restart your Mac
One of the simplest steps you can take is also one of the most effective. Restarting your Mac cures most slow-downs, because it forces background processes to quit, frees up RAM, and generally lets you begin afresh.
Check your Internet connection
For many Mac users, a slow Internet connection is synonymous with a slow computer. That's because almost everything they do--from surfing the Web to checking email--requires Internet access. If your online activity seems to be moving at a crawl, improving your connection can solve your problem.
Check Network preferences: Do you see a spinning beachball in Safari when you try to load webpages? It may not be clear at first whether pages are taking an unusually long time to load or whether your connection is actually broken. Select Apple menu > System Preferences, and choose Network. Click Assist me and then Diagnostics, choose your connection type, and then click Continue to run the tests. If a problem pops up, follow the Network Diagnostics tool's suggested advice for dealing with it.
Check your speed: If you can get the page to load, go to a site that tests Internet connection speeds. Since I use Comcast Internet service, I go to Comcast Speed Test. Other options include Speedtest.net and TestMySpeed.com.
You probably pay for certain upload and download speeds--look at your bill or check with your ISP to see what those speeds are supposed to be. If you detect a slowdown, call your ISP to ask if it is experience general problems that may be affecting you. If so, you'll have to wait for someone else to correct the issue. Alternatively, your ISP may offer to check and perhaps fix your modem connection via remote commands.
Test all your Macs: If you have more than one Mac, test the Internet speed on all of them. If the slowdown occurs on only one machine, the problem most likely originates with that Mac. It may, for instance, have an unusually weak Wi-Fi connection. If so, you may be able to put things right by turning Wi-Fi off and then back on. To do so, go to the Wi-Fi menu in the menu bar and select Turn Wi-Fi Off. Wait a few seconds and then turn it back on.
Disconnect and reconnect your modem: Unplug your Internet modem and wait for about 10 seconds. Plug it back in. If you have a separate router, such as an AirPort Extreme, do the same with the router. Wait for everything to reboot. Check your speed again.
Free up disk space
To function at a decent pace, OS X needs to have adequate free space on your startup drive. If your Mac's available disk space shrinks to near zero, the system will become very sluggish (as I covered in a Bugs & Fixes column).
Check disk usage with Activity Monitor: One way to check the amount of free disk space on your Mac is to launch Activity Monitor (included with OS X in the /Applications/Utilities folder) and click the Disk Usage tab at the bottom. My rule of thumb is that a system should have at least 10GB available (or 10 percent of its drive's capacity, whichever is smaller). If your Mac's free space falls below this threshold, you'll need to clear some additional space.
Search for large files: An easy way to locate big files that you may no longer need is to use Spotlight. In the Finder, press Command-F, click the plus (+) sign, click the Kind pop-up menu, and select Other. Select File size from the resulting list, and then click OK. Set up the criterion to read File Size is greater than 500MB. If nothing of consequence appears when you run that search, select a smaller file size and try again. Or try a utility such as Id-design's $13 WhatSize. Delete any files that you no longer need.
Delete cache and log files: Ordinarily, cache files, such as those in the ~/Library/Caches folder, help your Mac run faster. If you delete them as a quick fix, your system will rebuild them later. Log files, such as those in the ~/Library/Logs folder, keep track of past events on your Mac, such as past crash reports. Most users never look at these files, so you can probably delete them with no adverse consequences.
The simplest way to clean out your cache and log files is with a utility such as Onyx from Titanium's Software (donation requested). After launching Onyx, go to the Cleaning tab. For starters, delete only items in the User and Logs sections.
It's possible that a bug, such as a "runaway" log file, may be causing your drive to fill up much faster than it should. If your available free space returns to near zero shortly after you clear out the files, a bug of this type may be the culprit; if your Mac has this symptom, check the Web for possible solutions.
Check CPU usage
If your Mac's Central Processing Unit (CPU) is overwhelmed by an app, everything on your system may slow down.
Launch Activity Monitor and select My Processes from the pop-up menu at the top of the window. Next, click the % CPU column to sort by that criterion.
If an app consistently remains at or near the top of the % CPU list--and especially if it accounts for an atypically high percentage of the CPU's workload (probably anything over 50 percent, and certainly anything over 80 percent)--that app may be gumming up the works. To find out, select the app and click the Quit Process button.
By far, the most likely source of trouble is Apple's Safari (and more specifically, a webpage that uses Flash). If webpages are loading extremely slowly, and if the % CPU for Safari and/or Safari Web Content remains high, it's time to take action.
To reduce the odds that a Safari slowdown will recur, minimize the number of webpages you keep open at one time. You might also try using Google's Chrome: If one tab misbehaves in Chrome, you can use Activity Monitor to zap it without bringing down the whole app.
Check memory usage
Macs using hard drives depend on a combination of physical memory and virtual memory to get things done. Physical memory is faster. The more heavily your Mac has to depend on virtual memory, the slower it will perform. Virtual memory also creates swapfiles that increase in size over time. (To find these, select Go > Go to Folder in the Finder, type in /var/vm, and click Go.) Swapfiles can contribute to a system slowdown by using up disk space.
Quit apps: To improve matters, quit apps that you aren't currently using. Then restart your Mac.
Check memory usage with Activity Monitor: As before, to check for memory usage problems, launch Activity Monitor. Look under the column headers Real Mem and Virtual Mem. If an app is using a disproportionate amount of Real Mem and Virtual Mem, you can quit it by selecting it in the list and clicking Quit Process.
I generally focus on the System Memory statistics at the bottom of Activity Monitor. (Click the System Memory tab to see these.) If the 'Page outs' and 'Swap used' values are high (over 2GB, as a rough approximation) and the amount of free memory in the Free listing approaches zero, insufficient memory is probably contributing to your slowdown.
Deal with persistent problems: If speed and memory problems remain, or soon reappear, one of two things may be responsible. First, the problem may be a "memory leak"--a bug that causes a particular app to use excessive amounts of memory. Often a Web search will confirm this situation and offer further advice. Second, your Mac may not have enough installed memory to meet your current needs.
Money is the ultimate cure for stubborn slowdowns.
Add RAM: If your Mac doesn't already have its maximum amount of memory installed, and if its memory is accessible for upgrades, adding RAM is the quickest and cheapest way to add zip to your Mac. For more information about adding RAM, see Apple's guidelines for the MacBook Pro, the MacBook, the Mac Pro, the Mac mini, and the iMac.
Get a bigger hard drive: If you continually bump up against your hard drive's space limits, consider replacing it with a larger-capacity drive or offloading some of your files to an external drive. Shifting from a conventional hard drive to an SSD drive can speed things up, too. The site Ifixit contains useful step-by-step guides to replacing the hard drive on many Mac models.
Buy a new Mac: If you can afford it--and especially if your Mac is more than three years old--your best bet might be to buy a new one. A new Mac is likely to have a faster CPU, more memory, a larger faster drive, faster ports (such as USB 3.0) and improved Wi-Fi performance. Add it all up, and it equals no more slowdowns, until the cycle repeats itself in a couple of years.