The straight answer is that activity monitor cannot directly help speed up your Mac. However, it can be used very effectively in an indirect manner to help identify problems that are slowing down your Mac. So, using activity monitor is for diagnosing performance problems that are affecting your Mac.
Once you find out those issues, fixing your Mac’s speed issues is sometimes as simple as just temporarily shutting down a Mac or maybe reinstalling your Mac OS.
Let’s look at an example, as suggested at http://www.whyismymacsoslowallofasudden.com. You were working on Adobe Photoshop, doing some very important graphic editing work on a wedding photograph for a client. During some important rendering tasks, your Mac just begins to freeze up, hardly responding. It gets to a point where you can’t even see your mouse cursor move. What do you do?
First, don’t panic. Don’t start pressing a bunch of keys. Remember, the more you press, the harder you are going to make your Mac work, potentially sending it into a crash from which it can’t recover, until you restart it. That way, you probably won’t be able to save your work.
So, first give it a few moments to breathe and see if it comes back to normal. If it still doesn’t, open Activity Monitor. If it doesn’t open up right away, it will eventually. Just give it some time. Once it opens up, take a look at the apps that are using very high system resources. Though in this case it sounds like Photoshop could be the culprit that is slowing down your Mac, you might be surprised to find that some simple background app like iMessages could be slowing down your Mac, due to some bug. If you find something like that, just shut down that problematic app with a mouse click and you will be amazed at how your Mac springs back to life, even when running a demanding program like Adobe Photoshop.