Try as you might, there is simply no avoiding times when your computer slows down. The likely culprit is many cases is your system memory being stretched to its limit. If you are on a Mac and would like to keep closer tabs on your current memory usage, there is Memory Diag.
Free for "a limited time" in the Mac App Store, Memory Diag provides a graphical representation of your current memory usage, spotlights any apps that are using a significant amount, and lets you optimize your Mac's memory resources with a click of a button.
When you install Memory Diag, the app places a small icon in the menu bar. Click on it to reveal a gauge that shows you dynamic memory stats: file cache, wired memory, compressed memory, and app memory. These are the same stats you can view in the Activity Monitor, but here they are easily accessed from the menu bar and shown in a colorful gauge. At the center of the gauge is an Optimize button, with a needle that shows you your current memory pressure. Listed at the bottom of the Memory Diag window are any offending apps, those using a significant amount of memory.
When you click the Optimize button, Memory Diag attempts to restore your Mac to the state after a system restart by emptying caches and lowering compressed memory. The developer, Rocky Sand Studio, notes such optimization speeds up your Mac in a couple scenarios: before and after running memory-intensive apps. Before running a demanding app, it'll clear the decks, so to speak, so an optimal amount of memory can be made available. And after closing a memory-intensive app, it'll speed things up by cleaning up the caches the app left behind.
Sometimes, the Optimize button will be disabled by a big exclamation point. The exclamation point shows up when you are in a particularly high-pressure situation, where if any more memory is compressed your system will slow down even more. If you've passed the point of memory optimization, your best course of action is to close the offending apps which are hogging memory resources.
There are two other buttons on the Memory Diag window. The small "i" button to the right of the gauge shows you how much memory is installed on your Mac, while the hamburger button in the upper-left corner opens lets you select one of three themes, change the orientation of the menu-bar button, automatically launch the app at login, and add an icon for it to your Dock.