I've encountered a similar problem with my Mac right after I started using Evernote. My first thought was that Evernote caused the problem, but then I noticed that the system slowed down in general while I was using other apps like Skype or Safari.
The reason might be that your system is overloaded with various apps and files. Also, most recent app updates require more RAM so old macs can slow down throughout couple of years. There are few things you can do:
1) Identify unused files and apps and clean your system yourself. There are some helpful instructions online, they'll help you save some time.
2) To save even more time, you can use one of cleaning tools that do major part of work for you. I used to clean up my mac with Cleanmymac and was pretty satisfied with results. Cleaning all the system junk by myself would take a couple of hours. There are alternative Mac cleaners - Dr Cleaner, Mackeeper, Macclean app, so you can choose the one you like best.
3) If that doesn't help, take your mac to some hardware service center and check your hardware health. If your mac is over 2 years old, chances are, you need to replace some of it.
Hope it helps!