Macbook Pro is Excruciatingly Slow
Troubleshooting is a process of elimination, so you will need to try a number of things one by one and take note of the results. 

Start in Safe Mode

First of all, try starting your MacBook pro in Safe Mode and testing to see whether you still suffer the same sluggishness in performing typical tasks. To do this, follow these steps:


Shutdown your MacBook Pro
Restart your MacBook Pro
Immediately press and hold the Shift key.
Let go of the shift key when you see the login window. 


Use the computer as per usual and determine if it's working faster now? Report back.

Run hardware diagnostics

You should also run some hardware diagnostics. To do this, follow these steps:


Shut down your MacBook Pro 
Restart your MacBook Pro
Press and hold the D key before the gray startup screen appears.
After a while, Apple Hardware Test (AHT) will start.
When prompted, select your language and click the right arrow. 
When the AHT console appears, you can choose to run Basic tests by clicking the Test button. I suggest you select the "Perform extended testing" checkbox before you click the Test button.
Your test results will appear in the window in the bottom-right of the console.


Note that the extended test will take some time. Take a note of the results and report back. 

For good measure, it may be worth resetting both your NVRAM and the SMC.

Reset your PRAM/NVRAM

Reset your Non-Volatile Random-Access Memory (NVRAM) by following these steps:


Shut down your machine. Yes, a full shut down, not just logging out.
Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
Hold those keys down until your Mac reboots again and you here the startup chime.
Let go of the keys and let your Mac reboot normally.


Note: When you log back in you may need to readjust some of your system preferences (e.g. mouse speed, time and date/timezone, etc).

Reset the SMC

For your model MacBook Pro, resetting the System Management Controller requires these steps:


Shut down your MacBook Pro
Keep the MagSafe adapter (power cable) plugged in.
Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button
Let go
Turn your computer back on with the power button.


Once you've done all these steps let us know how you went.