Why is my MacBook so slow?!
If you have asked (or screamed) this question to no one in particular in the past, there is a free Mac app that can help answer it. System Lens installs in your Mac's menu bar and gives you a rundown of the apps you have running, rating each on the amount of system resources it is using.
When you click on System Lens' icon in your menu bar, you'll see the apps you have currently running with a Low, Medium, High rating. The icon itself tells you how stressed your system is -- one bar means overall usage is low, two bars is medium, and three bars means things are about to boil over. In settings, you can change the icon's bar-rating system to a numbered system. And should you (logically) take issue with running an additional app in an effort to better manage a slow system, you can check a box in settings to disable System Lens from intermittently taking your Mac's pulse, which means the app itself will use fewer system resources. Lastly, if, like I did, you find most of the apps get a Low rating, you can tweak the threshold for the ratings in advanced settings.
System Lens not only monitors your system, but it also provides a quick and easy way to close any app that you deem too much of a resource hog. Click on an app from System Lens' drop-down window and an X button will appear where its rating was before. Click on the X to close the app. In my experience, System Lens is far more effective at closing apps than I am. Firefox, for example, almost always refuses to close when I first ask it to, but System Lens closed it instantly. Same for bloated iTunes and slow-as-molasses iPhoto.
System Lens provides a free and easy way to monitor and manage the apps on your Mac. The only issue I found with the app is that it didn't always show all of the apps I had running, no matter if they were minimized or open and running right there in front of me. Just for its ability to close apps instantly, however, it's worth checking out.