A recent article on KernelTrap highlights the high cost of supporting atime ("last-accessed time") updates on Linux file-systems. It has been suggested that desktop machines should just mount their file-systems using the "noatime" option to avoid this overhead.
Each time you read a file, its atime has to be updated. This can quickly become costly if you have applications that access a lot of small files. Most modern desktop environments, office suites, compilers (think of C/C++ headers), browsers, etc. fall into this category, so Linux takes a lot of unnecessary performance hit for data that is of interest only to a very small set of applications like tmpwatch. (Apparently even Windows has the same issue with NTFS.)
I have now changed the "/etc/fstab" on my PC to mount its file-systems using the "noatime" option. It does seem to have slightly improved the responsiveness of the desktop, though this could just be a placebo effect. On the other hand, in the KernelTrap article people have presented measurements that demonstrate the actual performance improvements brought about by using this option.