Q: I have a MacBook Pro Early 2011 Running Slow

sir-all I can suggest is that you do a backup of your stuff, see if you can create a usb installer of Yosemite, erase and reformat your SSD, maybe adjust TRIM (don't know if it's needed on Apple SSD or not) and try to reinstall Yosemite. Also, upgrading your ram from 4 gigs to 8 gigs (maybe even 16 gigs) would be helpful, especially if you like to run lots of applications at the same time. It's easy to install it yourself. As far as I know the maximum amount of Ram you can install in  your Macbook Pro early 2011 is 16 gigs (8 Gig sticks x2) and may help you run Yosemite better. As for your CPU running on 97%, find and run Activity Monitor which should tell you what things are running in the background or at startup, and help you to quit them. 

Perhaps buying a bigger SSD would be a good idea, too. 320 Gigs seems a bit small to me...

that's all I have

JB

Did you actually erase the internal disk before you reinstalled OS X? After the reinstall were your user files & Applications still on the disk?

It might help if you go into more detail…

What process is using 97% of CPU?

How long ago did you reinstall OS X?

What other steps have you taken & do you have a full bootable backup of the internal disk?`

If you have clean installed to the internal disk & it is still slow then that indicates a possible hardware error.

It is simplest to take the Mac to an Apple store & get them to check the hardware. They may be able to test it for free, and then give you a quote for any potential repairs.

Booting from a USB disk will always be slower than booting from the internal disk but the CPU usage should return to a normal level after it boots & settles down.

If you have the original grey installer disks you can try booting into Apple Hardware Test. One of the install DVD's will have the instructions printed on it in white text. Insert that disk & use the startup command (normally hold D on startup).

Run Apple Hardware Test several times on normal & extended mode, record any errors (you may need to photograph the screen).

Using Apple Hardware Test - Apple Support

If no errors appear try enabling 'loop mode' (press cmd+L inside Apple hardware test). Allow the tests to run for several hours to see if an error appears.

Apple Hardware Test may not find all errors, but it can give you a good place to start looking for more info.

You could also see if the OS works any better in Safe mode…

Try safe mode if your Mac doesn't finish starting up - Apple Support

Note the performae in safe mode will be worse than a normal boot, however you should test as much as possible to see if it works OK.

Limited graphics performance in OS X recovery or safe mode - Apple Support

Some other general tests & resets to try…

How to Reset NVRAM on your Mac - Apple Support

Resetting the System Management Controller (SMC) on your Mac - Apple Support

Repair the disk

Boot into recovery mode (hold cmd+r at startup)

Open Disk Utility from the list of options

Select the main boot volume in the lefthand list (normally named Macintosh HD).

In the First Aid tab, repair the disk & make a note of any errors.

Also repair disk permissions - this will have many errors that can be ignored

Mac OS X: Disk Utility's Repair Disk Permissions messages that you can safely ignore - Apple Support

Please post info for any error messages you see, it helps if you can copy the exact text, screenshots are OK.

kernel_task is where the OS communicates with the hardware. It suggests a possible hardware issue.

Can you run Apple hardware test? See the my last post for more info on that, please try the other suggestions when you have time.

Also open /Applications/Utilities/Console & see what messages appear in the 'all messages' section when the Mac is slow & kernel task is busy, please select a few messages - posting too many may not help.

You have the original installer DVD inserted into the Mac? Is that the grey disc with the small instructions printed on the disc?