There are several different things that could cause your computer to run slowly. It sounds like it starts off normal but slows down. I would first look at items that might start automatically start when you start your computer and removing the items that you do not want to start automatically:
Permanently prevent an item from opening automatically when you log in
- Choose Apple menu > System Preferences, and then click Users & Groups.
- Select your user account, and then click Login Items.
- Select the name of the item you want to prevent from opening automatically at login.
- Click Delete "-" below the list on the right.
You can find the full article here:
After you remove those items, you will have to restart the computer to test it out. If it is still running slowly after that, I would use this next article:
If you are experiencing the sluggishness in one application, you can follow the section under "If the issue occurs in one application only." If it doesn't matter what application you're in, follow the steps under "If the issue occurs in various applications, or in Mac OS X."
You can further isolate the issue using this following article:
Thank you for posting in the Apple Support Communities.