Your Mac was running fine. It started up quickly, and was zippy when you worked and snappy when you played. But suddenly something’s wrong: when you start it up, it takes longer – maybe a couple of minutes, maybe more – than it once did. What’s slowing things down?

There can be a lot of reasons your Mac is slow to start up. Here are the troubleshooting steps you should take, in this order, to find out what’s wrong and fix it.

Hardware peripherals
The first thing to check is any hardware connected to your Mac. Sure, it might have been working fine before the slowdowns started; unfortunately, electronic devices can go wrong.

Disconnect every external hardware device but your mouse and keyboard: that includes hard drives, printers, scanners, extra input devices, hubs, network cables and even cables that connect to your iPod. Restart your Mac.

Startup still slow? If you have a spare mouse and keyboard, disconnect your current input devices, connect the spares and restart. If the problem persists, and if you use wireless AirPort networking, select your internet connection in the Network preference pane and make sure the correct DNS servers are listed in the DNS Server box; they should be the ones specified by your ISP.

Let’s say you’re lucky – one of these steps resolves the slow startup. You can then figure out which device is the problem by reconnecting your peripherals one at a time and restarting after each addition. If and when your startup chokes, the last peripheral you added is the problem.

Choose an account in the left column to see all of its login items. Hover the cursor over an item, and a tooltip will appear, showing its path

New software
If peripherals aren’t the problem, your next suspect is new software. Did you add any programs recently? Did you update Mac OS X or any of its components such as QuickTime or Safari, or do any other system update that required you to restart your Mac? Did you update any drivers (software that runs a device such as a printer or scanner)?

Should the answer to any of these questions be yes, the first thing to do is uninstall the suspect software. For a recently added application, you can try to remove it using either the uninstaller that came with the program or with a utility such as the AppZapper (www.appzapper.com).

Removing updates to the operating system or its built-in apps (Safari and others) is trickier. If you have a recent backup of your system, such as a Time Machine backup, you could try restoring the entire system to the state it was in before you installed the suspect software. Don’t just restore individual folders (such as System or Library); you need to restore the whole thing.

Again, go step-by-step: uninstall all your new software, then reboot your system. If startup goes faster, add the new software back one piece at a time, rebooting between each reinstall, until you identify the culprit. If removing all that software doesn’t help, move on to the next step.

What’s loading?
When neither hardware nor new software is causing the problem, your next suspect is software that loads (or should load) on your Mac at startup.

To begin, reboot your system, then press and hold the shift key just after you enter your user name and password at the login screen; this temporarily disables login items. (If your Mac is set to log in automatically, hold down the shift key when the blue screen appears.)

If this accelerates your startup, click on the Login Items tab. Scan the list. If you’re sure you don’t need an item, select it and click on the minus-sign (–) button to delete it. Then reboot.

Should the problem persist, next check items that load at system startup (as opposed to your own individual login). Most such items are in the /Library/StartupItems folder. Restart your Mac, and hold down the shift key as soon as you hear the startup chime; this tells OS X to do a Safe Boot, meaning it’ll load only essential kernel extensions.

If your Mac starts up quickly in Safe Mode, move the contents of the StartupItems folder to the desktop, then try restarting to see if that resolves the problem. If startup goes quickly, you know something in that folder is causing the problem; add back the items in it one at a time until you replicate the slowdown. Caution: Do not move anything from the /System/Library/StartupItems folder.

Disk problems
The next potential problem is a hard drive that isn’t mounting correctly.

Open Disk Utility (in the /Applications/Utilities folder) and use it to check all of your hard drives: select a disk in the left column, click on the First Aid tab, and then click on either Repair Disk (for a nonstartup disk) or Verify Disk (for a startup disk). If Disk Utility says that the startup disk needs repairs, you will have to boot off your Mac OS X installation disk. From there, run Disc Utility from the Utilities menu.

If Disk Utility finds errors on other disks, it may be able to repair them, but in some cases it won’t be able to. In that instance, you should use a program like DiskWarrior (www.alsoft.com), which can repair many disk problems that Disk Utility can’t handle.

Use Disk Utility to check your startup disk for problems. If it finds any, reboot using your OS X install disk and run Disk Utility from there

System profiler
If a hard drive isn’t causing the problem, other built-in hardware could be. To make sure your Mac’s innards are healthy, go to the Apple menu and choose About This Mac, then select More Info to open System Profiler. Click on Hardware in the Contents column and check the information shown there.

If the amount of RAM isn’t what it should be, some memory modules may be unseated. In that case, you’ll need to open your Mac (if you can) and make sure your RAM chips are firmly ensconced in their slots. Alternatively, you could take it to a nearby Apple Store or other reputable repair shop. If System Profiler reports the wrong number of processors, you’ll need to have your Mac checked out professionally.

Look in System Profiler to make sure that your Mac’s hardware is shipshape

Checking logs
If you’ve gone this far without resolving the problem, it’s time to start checking your Mac’s logs. You can view them in the Console app, located in /Applications/Utilities. (For more on tracking down trouble with Console, see www.macworld.com/3302.)

In particular, look at all the log entries that occurred just after startup. Click on All Messages in the left column, and scroll up until you find messages from localhost kernel at the time you restarted your Mac. Scan the log from this point on for any error messages suggesting that software did not load correctly because it couldn’t be found. (Those messages shouldn’t be too hard to spot; they’ll say something like “could not load”.)

If you discover that none of these suggestions solves your startup woes, search Apple’s support site (www.apple.com/support) for “troubleshoot startup.”