My Imac became very slow

Question

Hi,
I have bought a used Imac
I have updated memory and hd.
It works fine most of the time but
sometimes it becomes very slow and I see the colored ball in between operations.
Could anyone help me find out what is going on.
Thank you
Jean-Pierre Sainfeld
Model Name:          iMac
I have updated memory and hd.
  Model Identifier:          iMac8,1
  Processor Name:          Intel Core 2 Duo
  Processor Speed:          2.8 GHz
  Number Of Processors:          1
  Total Number Of Cores:          2
  L2 Cache:          6 MB
  Memory:          4 GB
  Bus Speed:          1.07 GHz
  Boot ROM Version:          IM81.00C1.B00
  SMC Version (system):          1.30f1
Vendor:          Intel
  Product:          ICH8-M AHCI
  Link Speed:          3 Gigabit
  Negotiated Link Speed:          3 Gigabit
  Description:          AHCI Version 1.10 Supported
Hitachi HDS723030ALA640:
  Capacity:          3 TB (3,000,592,982,016 bytes)
  Model:          Hitachi HDS723030ALA640                
  Revision:          MKAOA580
  Serial Number:                MK0311YHG7P7UA
  Native Command Queuing:          Yes
  Queue Depth:          32
  Removable Media:          No
  Detachable Drive:          No
  BSD Name:          disk0
  Rotational Rate:          7200
  Medium Type:          Rotational
  Partition Map Type:          GPT (GUID Partition Table)
  S.M.A.R.T. status:          Not Supported
  Volumes:
  Capacity:          209.7 MB (209,715,200 bytes)
  Writable:          Yes
  BSD Name:          disk0s1
ephraim3tb:
  Capacity:          3 TB (3,000,249,008,128 bytes)
  Available:          2.69 TB (2,687,080,443,904 bytes)
  Writable:          Yes
  File System:          Case-Sensitive Journaled HFS+
  BSD Name:          disk0s2
  Mount Point:          /

Answer 1

Did the previous owner erase and reinstall the Mac OS X?
Make sure there's enough free space on the hard drive.
Right or control click the MacintoshHD icon. Click Get Info. In the Get Info window you will see Capacity and Available. Make sure you have a minimum of 15% free disk space.
The Spinning Beach Ball of Death
Hopefully you were able to get the original install discs that came with the Mac so you can verify and if necessary repair the startup disk.
Using Disk Utility to verify or repair disks