Constant SBOD

My iMac (intel with 4GB RAM) gets slower and slower during the day so in the evening its terrible. This message took ages to write because it can't seem to cope with even simple tasks and the SBOD is my constant companion!. I've been regularly clearing trash, doing first aid on disk utility and working with one application open at a time where possible. I can hear a faint whirring sound when it seems to be "thinking". Can anyone help please?

This may find faulty RAM or other hardware related problems.
Using Apple Hardware Test

This will boot your system with no third party products. Good for ruling out non Apple causes.
OS X: What is Safe Boot, Safe Mode?

You could also try the rest of the OS X 101 troubleshooting like creating a second user to see if the problem follows the user etc etc etc.

1. How much free space on your drive? A full drive can cause slowdowns as the system tries to write memory to the cache.
2. How many background things are running? Look in Applications/Utilities and run Activity Monitor to see what's running.