Mac working extremely slow

Question

I have MacBook Pro from 2011 year with 8 GB RAM.
I changed hard drive to new one, with 7200 rpm and 16 MB cache, and fully reinstalled system from ESD disk (not from any backups). And the system working extremely slow, like running Windows 2000 on 486 computer.
It is difficult to write text, and actually impossible to make so easy things, like web surfing.
Here is activity monitor picture: actually nothing is running, but CPU full.
PS. Some times, I can see, that kernel_task consumes >300% of CPU.
Looking for advises.
Thank you.

Answer 1

There are quite a few reasons a computer will start to feel slow but the two most common are: an overly full hard drive and not enough memory.
If your computer seems nice and peppy when you first boot it up and then seems to slow down as it is used, memory is likely the culprit. Similarly, if you notice it slows down as you start up another program, the problem is too little memory (or too many programs in use). If this is the case adding more RAM is your answer.
Because programs and the operating system use the hard drive for temporary storage I've always recommended that users keep a good chunk of their hard drive empty. Drive sizes vary tremendously making it harder to recommend a specific amount but if less than 5% - 10% of your drive empty you may not be leaving enough room for virtual memory and various important cache files which can cause a computer to slow down. Move some files to another drive and see if that helps (media files are good candidates for removal since they tend to be large).
These aren't the only reasons a computer might be slow but they are the two I start with.