Any speed increase you notice is not due to any update included with OS X 10.6.3. It is simply a perceived speed increase.
The Mac OS X 10.6.3 update does not include any kind of update to any portion of OS X that would cause any kind of general speed increase, across the board, with your computer. None of the updates included in 10.6.3 has the intention of making things faster -- it is, for the most part, a bunch of security and stability updates.
It is possible, like djackmac said, that some process the 10.6.3 updater ran before/during/after the updating process caused some kind of anomaly with your system to be fixed, some storage that needed optimizing to be optimized, or some scheduled process that was supposed to be run to run.
10.6.3 was not intended to speed up anything with your computer -- any kind of general speedup you're experiencing is either perceived or some kind of side-effect.