Windows

The answers are:

Even when cleaned, the registry grows in size and consumes more resources.

Programs are updated from the "image" and the newer versions use more resources.

People install things they shouldn't.

New drivers eat more memory than old ones.

Hot fixes and security updates eat more memory.

Just as an experiment, take a snap shot of the system before you image it.

Then compare the image registry and the "has issues" registry. This may give you a very good idea of what programs are different, so you can track down the source of the issue.

Best of luck.

Over time with installing sloppy "Windows" products, Patches and uninstalling/reinstalling crap to see whether you like it or not, the System ends up messed up. Especially the Registry.
There are some good Cleaners out there but not one of them will give you a pristine Repair.

Service Packs, Antivirus updates, Security updates, Temp files being created and deleted, a lot of information being written and re-written to the Hard drive. FAT corruption isn't always evident. Unless you regularly run chkdsk and a Defragmentation tool.
Users logging on using the System and logging off all takes it's toll.

Another factor is wear and tear on the equipment as PSU's age they lose their functionality. Not being able to produce the right Voltages can have detrimental effects on the hardware.

There is a list of possibles and I don't think that it is one simple factor involved here. I'm sure that it's between the OS (Registry), Memory and the Hard drive though for simply being poor performance.