The cleaning of cache is always given to solve just about any problem you can thing you come across. Usually it only makes your Mac slower while the data that was cleaned out of cache is reloaded once again. It is more of an old wife's tale that a true solution to a performance problem.
How much free space do you have?
I agree with you, cache cleaning is not the way to speed up things as caches are created to enable fast access to data previously generated on many tasks. You can see on Activity Monitor that when a cache cleaning is done there will be some processes performing the cache rebuilding.
I would be happy to tell you that a clean erase and format, followed by an brand new OS X installation and recovering only your documents data would bring back that boot speedy you once have seen, but I did try this with no success. I remember having a ~20 seconds enabled use, today I have to wait ~60+ seconds to see User Interface and if I try to use right away I am not successful, some ~15 seconds more are necessary.
I have only Snow Leopard fonts, I have no Login Items apps and also have the Automatic login enabled - these were suppose to speed up the startup. As this did not worked out I just launched that 5 minutes Desktop Background change - slow and amusing.
Good luck, K
In Mac OS X v10.6, update_dyld_shared_cache(1) is not automatically run. The Installer runs it when you first install the operating system; Software Update runs it when it updates the operating system. If for some reason you swap out an system dynamic library, you should manually run update_dyld_shared_cache.
Restarting the computer in Safe Boot mode deletes the dynamic-loader shared cache and will rebuild on reboot.
If you suspect the shared cache is somehow corrupt, you can run:
sudo update_dyld_shared_cache -verify
which re-creates the cache and compares the result with your actual cache file.
SBOD symptoms have been corrected by rebuilding this cache. Yes the initial reboot will take longer as the cache rebuilds itself.
OnyX for OS X cleans not only that but a host of other things as well.
It's not as much as a performance booster as more of a privacy/corruption resolving solution than may or may not also resolve your performance issue.
Myths of required versus not required maintenance for Mac OS X for information.