Why is my MacBook Pro running so slow as of late?

[iMore Question]

My macbook is roughly 4 years old and I have had no problems until just recently. When I am browsing the internet, sometimes websites never load even though my internet connection is strong. A couple of times, restarting my computer would fix this process, however, just recently restarting my computer has become more of a process than actually waiting for the sites to load if they ever do. I am just wondering if it is necessary to back-up my computer to an external hard drive or what some other options may be. I honestly don't know anything about my computer, or computers in general.