Toshibitsu is right, the dust you accumulated in 4 years is most probably thermal throttling your processor. It means that your processor can't cool down and is forced to reduce its bus speed to avoid frying.
In my opinion you should remove the bottom pannel and spray using a air can. There aren't many risks as long as you take your time.
---------
What you need to know:
-obviously turn off the computer
-buy a screwdriver (i assume it's a pentalobe but check it on google)
-unscrew these screws
http://i.i.cbsi.com/cnwk.1d/i/tim/2011/06/29/MacBook_Pro_bottom.jpg
-remove the pannel
-now i'd recommend to unplug the battery just to be cautious
https://d3nevzfk7ii3be.cloudfront.net/igi/2K2I1YfoddnJ6CB6.medium
-and now you can spray the shit out of it (never turn the can upside down while spraying, better move the pc around than the can. Also, when you spray the fans, hold them with your hand so they don't spin otherwise you could damage them)
Also about the hdd i assume you have a macbookpro retina if it's late 2013 so there is no hdd but a m.2 ssd inside which is less likely to fail or to be the cause of your fps loss.
You can also try to reformat your disk and reinstall osx if you've used it since you bought it.
Anyway, even with all that i don't think you will get the same amount of fps as when you first had the computer.
Hardware just slowly reduces it's power with time + the games you are playing must have been updated overtime and became more hardware demanding.
good luck!
Facebook
Twitter
Instagram