I am Pete, and I'll be happy to assist with your question today.
Firstly, how much free space is there on the hard drive of your MacBook Pro?
Nothing slows down a Mac more than having too much stored on your hard drive.
If you can free up some space, you’re sure to see an increase in speed.
Cleaning up old movie files, organizing your Desktop, and searching your Mac for old files you don’t need is the easy part. However, the hard part is cleaning up your system, and figuring out all the stuff that can be safely removed for more space. The best option is to use a Mac cleanup tool, like CleanMyMac 3. It cleans up your entire Mac, the easy stuff and the hard stuff:
Having the latest version of OS X is good practice (the latest version right now is OS X El Capitan). Don’t think of it as adding more junk to your already slow Mac — think of it as new software that makes your Mac run more smoothly. So, update your Mac’s OS X to El Capitan to help speed it up.
Another possible cause of a slow Mac is that disk permissions are broken. Again, you can repair them with CleanMyMac 3. Download the app and go to Maintenance tab, click “Repair Disk Permissions”.
When you start your Mac, a lot of things load up in the background. Not only do they slow down your startup, but they continue to do so the whole time you’re on your Mac.
You can lighten the load on your Mac by removing unnecessary apps. Go to your System Preferences > Users & Groups, and then click on your username. Now, click on Login Items, click on the name of an application you don't need to launch during startup, and then click the "-" symbol located below the list to the left; this will remove the application from the list — the fewer applications on the list, the better. This should help out a ton with speeding up your slow Mac.
Finally, using Activity Monitor will show you what processes are using up all of your system resources. Quitting an app that’s taking up a lot of processing power could make a huge difference in speeding up your slow Mac. Open up your Applications folder and then your Utilities folder. Here you’ll find the Activity Monitor, open it. Check out the list of apps and processes that happening on your Mac in real-time. From here you can see what’s causing trouble with your Mac. Click on the Memory tab at the top, then the Memory filter at the top of the list; this sorts the programs by the amount of space they’re taking up on your Mac RAM. The higher the number, the more power they need. Stop an app from operating by clicking on the app in the list and then clicking the gray “x” icon located at the top-left corner of the window. Don’t remove anything you don’t know though.
Does that all make sense, or is there anything else I can help you with?
I think you should try what I have suggested before posting such harsh criticism.
You need to follow ALL of these steps, not just the few you carried out previously.
A slow computer causes these startup problems, so you need to resolve all of the points I have mentioned.