Q: My iMac is not as fast as just one year ago

Should it be defragmented?

No. X does the auomatically.

Some semi-randomn thoughts on addressing performance problems.

Activity Monitor
Look at what is happening with your Mac when you run Activity Monitor.

Macintosh-HD -> Applications -> Utilities -> Activity Monitor
Click on the CPU tab on the lower half of the window. Make sure the little triangle is pointed down.  Make the triangle point down by clicking.
With the down pointed triangle, you will see the apps which are using the most of the processor.

You can gain some understanding of Activity Monitor by  looking at it every once in a while.  Look at the small graph.

Here is how I have my cpu display set up:

AppleJack
"AppleJack is a user friendly troubleshooting assistant for Mac OS X. With AppleJack you can troubleshoot a computer even if you can't load the GUI, or don't have a startup CD handy. AppleJack runs in Single User Mode and is menu-based for ease of use."
http://applejack.sourceforge.net/

Console
You can use the console application to examine Mac OS log files.  The logs will contain information on various system processes.

Macintosh-HD -> Applications -> Utilities -> Console

File > open console log
this will display the startup messages

and
file > open system log
once the system is started, messages will go into the system log

Other folks
bdaqua advises running disk utility from install disk and performing a safe boot:
http://discussions.apple.com/thread.jspa?threadID=2256611&tstart=0

Allan Jones  advises running automatic maintenance scripts and checking for free disk space:
http://discussions.apple.com/message.jspa?messageID=10845311#10845311

K Shaffer    advises checking for available RAM and checking for free disk space:
http://discussions.apple.com/message.jspa?messageID=10879449#10879449

Run activity monitor.

See:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.