Must ask before moving forwards - No Admin Account on this computer.
Hum - is this computer Owned by a Company, University, School or Organization ?? If Yes, the computer may have been Enrolled in a Mobile Digital Management program ( MDM for short ). If the answer is Yes, one would have to contact the IT Department of the Company to fix this issue. They will control everything that is installed or not allowed to be installing on this computer and we can do noting about this - only they can.
If Owed by the user - then that changes things a lot. The Internal Drive is a slow Rotational / Mechanical drive and will always be slow when compared to Solid State Digital ( SSD ) drives. So, even after we try to fix things, it is going to be slow.
To create an New Admin Account - Credit goes to this well Respected Contributor @ BDAqua from this Posting
The exact steps are as follows >>
>>> Verbatim <<< See if you can create a new administrator account by restarting the Setup Assistant:
- Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
- Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
- Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
- Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.
- Reboot by typing reboot then ↩ enter.
- Complete the setup process, creating a new admin account.
Be very careful to notice the spaces in those Terminal Commands.
Once you've done that the computer reboots and it's like the first time you used the machine, except all your data will still be there. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!