Within Applications, then Utilities, you should have a program called Activity monitor.
Open this, then right click on the symbol in the dock and change the dock icon to "Show CPU usage". This is quite helpful to see what programs are maxing out your CPU at a glance.
Also while you are within Activity Monitor, select "Memory" and look for "Swap used" at the bottom.
If you are maxing out your RAM at any point, it will be using part of your hard drive as RAM. This keeps the computer running, but can slow down the computer massively.
You can also get an idea as to what programs you can close to free up RAM. Some internet tabs - especially websites like Google Maps and YouTube can really eat into your RAM.