When problems happen on any computer there are some basic troubleshooting steps that you can take. Unfortunately, many times people have no idea where to start, and problems may seem to be vastly different. For instance, sometimes specific functions, features, or settings of the system may not work properly, or applications may crash, or the computer may just be running slowly and lag all the time.
When these problems happen, there some basic steps you can take to start troubleshooting the problem. Even if you cannot isolate and fix the issue, performing these steps and relaying what you see to a technician, Apple "Genius," or anyone else helping you may greatly speed up the process and get the problem addressed.
One of the very first things you should try if a problem starts repeatedly occurring on your system is to boot into Safe Mode by restarting and then holding the Shift key after you hear the boot chimes. Doing this turns off unnecessary system extensions and loads the system in a minimal state. Safe Mode also runs a few system maintenance routines that may help clear problems. Once the system has fully booted in Safe Mode, try rebooting normally.
If the problem goes away when booted in Safe Mode but persists when the computer is booted normally, then the problem has to do with a third-party or nonessential system extension you have loaded on the system. Many applications can install these, including virtualization tools, drivers and hardware managers, and firewalls and security tools.
After attempting a Safe Mode boot, you can try troubleshooting peripheral devices attached to your system, such as external hard drives, scanners, printers, and secondary displays. Disconnect these devices fully from your system (as opposed to merely shutting them down) and reboot to see if the problem goes away. If it does, then you can test each device independently to see if having it attached to the system (by itself or in combination with another device) results in the odd behavior.
Many times specific applications will interface with certain devices, such as scanners and image processing tools. In these cases, you can try using Safe Mode in addition to removing all devices except the one that application uses, to test whether the application interfaces better with the peripheral when running in a minimized boot environment.
General maintenance routine
Sometimes slowdowns and odd behavior can happen because the various temporary files (caches) that the system builds for efficiency become corrupt. When this happens, cleaning these out can be an easy way to rebuild them and thereby speed up the system again. To do this, run a general maintenance routine to fully reset software setups like caches as well as hardware configurations like the PRAM and SMC settings. Resetting these will not harm the system, but may require you to set things like the mouse tracking rate and system volume again.
New user account
Many times problems in the system have their roots either in global resources (resources the whole system accesses), or in a specific user account configuration. OS X separates these by putting global items in the /Macintosh HD/Library/ and /Macintosh HD/System/ folders as opposed to the /username/Library/ folder.
An easy way to test this is to create a new user account on the system and log in to it to see if the problem persists in this account. If not then it is very likely the issue involves a configuration setting in your main account, and you can try clearing configuration files, preference files, and other settings in that account to fix the problem.
While you can use an existing secondary account to check the problem, existing accounts may share similar configurations (especially if they were upgraded, migrated, or restored from previous systems or backups). Creating a new account ensures that the account and its settings are as fresh as the current system can make them.
If an application is using system resources, especially if it is experiencing bugs that cause it to eat up memory or CPU cycles, then other programs can suffer slowdowns as the system tries to compensate for the rogue application. To check for available system resources, launch the Activity Monitor utility (in /Applications/Utilities/) and at the bottom of the main Activity Monitor window (press Command-1 to show it if it's not present), click the CPU, System Memory, and Disk Activity tabs to check those resources on the computer. If any of these sections shows high usage, then you can look to see if a specific program is causing the high activity.
To do this, at the top of the main Activity Monitor window click the "%CPU" column header to sort processes by CPU usage, and if one is showing persistent high CPU usage (especially if it's at or near 100 percent) then try shutting it down. If the program is not responding and its name is shown in red, you can try force-quitting it to shut it down.
In addition to checking CPU usage, sort the listings in the "Real Mem" column to see if any are taking a large amount of RAM, and quit them. One culprit that may take up a large amount of RAM is Safari, but sometimes background system processes such as "mds" or "mdworker" may also take a large amount of RAM or CPU if they are not working properly.
In addition to Activity Monitor, the system will show process output (including errors and warnings) in the system console and system logs. These can be exceptionally useful resources for determining if applications and system processes are crashing or not working properly.
To access the system logs and console output, go to the /Applications/Utilities/ folder and open the Console program. In here you can select the All Messages section to view live output from running processes, or look up recent crash logs in the System or User Diagnostic Reports sections.
Even if you do not know what to look for, if you see a specific warning or message output to the console when the slowdowns occur then that information may be useful to a technician, and you can easily copy it into an e-mail if needed.
Uninstall or disable features or applications
If the problem is not found to be in the account configuration, then you can next try uninstalling applications or disabling any other recent changes you might have made to the system. This is particularly true for any add-ons, program updates, or new applications you have added.
Unfortunately OS X does not come with an uninstaller utility or other application management tools, but you can use tools like AppZapper to root out and remove applications that did not come with an uninstaller. Alternatively you can contact the developer to ask how to fully remove the program.
If the problem happened right after an OS update, then your only options for uninstalling are to revert to a prior system backup (Time Machine backups are great for this), or reinstall OS X fully and update to the latest version again. This can be done using the OS X installation disc (or recovery drive for Lion).
Once again, these steps alone may not fix the problem for you, but they will help you isolate the issue and see if it is happening because of a specific device, account setup, system process or application you have running, or some underlying system extension or resource. Even if you cannot clear the problem yourself, the information you get from these steps will be useful for others who help you address the problem.