What can I do about constant beachballing and slowness?

Question

For the past four months my computer has been painfully slow. It beachballs constantly and for long periods of time, especially in Safari (not sure about other browsers). Sometimes all of the windows I have up go blank and appear to refresh themselves. The CPU usage is high at random intervals even though I don’t have much running. My laptop is running Snow Leopard and is almost 4 years old, so I understand slowing with age, but I feel like something else is wrong.
I still have the OS software but I'd prefer not to lose everything by reinstalling.

Answer 1

Let’s start with gathering some basic info.
(I think the following sequence works on Snow Leopard.  If not it is close and you can futz to find the info.)
Which MacBook do you have?  Please report back the season, year, and model identifier for your Mac.
 > About this Mac
          Note the specific version number of the OS.
More Info…
          Note the season and year.
          Note the amount of RAM.
System Report… > Model Identifier:
          Note the Model Identifier.
Storage
          Note the total amount and the amount used.
Post this information back.
I recently rejuvenated a 2009 MacBook from slow to quite fast with a combination of an OS reïnstall and hardware upgrades.  With the RAM upgrade the applications went from a long time to start to staring almost instantly and the boot time improved by 2x with an 8 GB RAM upgrade (~$100) and another 5x with a clean install and an upgrade from a disk to an SSD.  Let’s see how fast your Mac will go with some system maintenance and then you may want to consider RAM and/or disk upgrades.
In general, slowness can also be caused by incompatible 3rd-party software or an unhealthy SSD/disk.  Four months ago did you download any 3rd-party software or plugins?  If so that may lead down the software compatibility route.  Otherwise it may be a disk and/or RAM issue.
Over time one accumulates more 3rd-party plugins that take more memory.  If you have 1 or 2 GB of memory then more RAM can really help.  Your machine “almost four year old” Mac would make it a 2009 model which may hold up to 8 GB of RAM (a ~$100 upgrade).  I can check that when you report back with the specific system info.
You mentioned not wanting to lose your data.  Make sure you have a current backup.  You should always have a current backup anyway and it is particularly important if your Mac having problems.  If you do not have a Time Machine or other backup, create one before you proceed.  You will need an external drive you will dedicated to the backup.
Time Machine Basics: http://support.apple.com/kb/ht1427
Most commonly used backup methods: 
https://discussions.apple.com/docs/DOC-3045
Nothing in the following instructions should alter your user data but you should always have a backup before making major changes such as OS installs and other changes.
Hardware:
Check the health of your disk using Disk Utility
Finder > Go > Utilities > Disk Utility
Select the upper left disk icon > Look at the S.M.A.R.T. status.
If it does not say “Verified” then the SSD/disk will need to be replaced.  Definitely backup your system.
Select the volume icon slightly indented to the right usually labeled “Macintosh HD” > Verify Disk
If it finishes with "The Volume XXX appears to be OK.” then the filesystem is healthy.
See http://pondini.org/OSX/DU6.html for pictorial instructions.
While there > Repair Disk Permissions.
          Don’t worry about lots of warnings or error messages from repairing disk permissions.
If Verify Disk reported errors in red and/or did not finish with the green OK message, then repair the system disk.
Boot from the Snow Leopard install disk.
          Insert the disk.
          Boot - hold down the C key.
Use Disk Utility to repair the disk.
http://support.apple.com/kb/PH5836 steps 3 though 7.
If the repair fails the first time, run it once or twice again.
If the repair succeeds but the system is still slow,  reïnstall OS X.
Mac OS X 10.6: Reinstalling Mac OS X.
http://support.apple.com/kb/PH6606
Reïnstalling OSX will not effect your personal data.
However, have a backup, just in case.
Is the system running faster now?
Software:
Boot in Safe Mode.
http://support.apple.com/kb/HT1564?viewlocale=en_US
It will boot and shutdown slower than normal.
Does the problem go away in Safe Mode?
If so then there are Login Items that may be causing the problem.
 > System Preferences > Users & Groups > Click on your user name > Login Items
          Use “-“ to remove items you do not need.
Did that resolve the problem?
Try and SMC Reset.
http://support.apple.com/kb/HT3964
Create a new user account and login to it.
Is it faster in this other account?
If it is still slow run http://www.etresoft.com/etrecheck and post the results.  We are looking for incompatible software such as kernel extensions that may have slowed down the system.
For additional hints see Kappy's posting at https://discussions.apple.com/thread/5762378?tstart=0 .