As you might surmise there are a multitude of reasons for slow performance of a MBP. Without some specific information for the reason of a given problem. specific advice is difficult if not impossible to give. A forum such as this with its limitations in communications cannot hope to go through a multitude of possibilities.
I would suggest that you find a tech savvy individual and sit down with him/her and go over your MBP to identify the problem(s). That way you should learn a few things that will clarify some of the items in the link that I refered you to as well as possibly speeding up your MBP.
Rather than ask a dozen addtional questions for more details about the problem consider the following:
Apple Support Communities contributor etresoft wrote a very useful app to quickly gather certain system information that may help point to a cause of this problem. Go to his website, download and run EtreCheck:
Etrecheck will be in your Downloads folder. Open it from there. You may see the following dialog box:
Click Open - etresoft contributes to this forum frequently and can be considered a trustworthy developer.
It will take a moment to run as it collects its data.
Copy and paste its output in a reply.
Do not be concerned about anything that says "Problem" or "failed".
EtreCheck was designed to remove any personal information (such as your computer's name and serial numbers) but if you see anything that looks like an email address or any other personal information that should not be divulged to others, please delete or obscure that information when you post the reply.
When you are finished with EtreCheck, quit the program. It occupies very little space, and you can keep it or drag it to the Trash as you wish.