My 2011 MacBook Pro is not fast anymore, when I turn it on it takes 5 or more minutes to start using it. What can I do?
I suggest you run disk utility to correct the permission and other fault that may have occur during files moving delectation etc in the macintosh drive.
Applications> utility> disk utility/ select your Macintosh HD drive then click on "first Aid" button, then run. Let it run till the end, then rebook the computer 2 times to see if it speed up the boot time or not.
You mean, you are wanting to format your hard drive and reinstall the OS from scratch? If yes and you are comfortable with that, then you can do that. But really make sure that you know what you doing, back up your important files, applications dmg to another HDD, and follow tutorials on line to format and reinstall your computer.