Not quite true...
I think that iOS 6 will make your iPod Touch slower. iOS 6 was not designed for the iPod Touch 4g, so it will not run as smoothly as the 5G.
iOS 6 was designed for the iPod touch 4th gen, otherwise Apple would not be releasing it as an update for that model. Apple has tailored the code and functionality of iOS 6 to the ability of the device, by excluding some features and possibly by including device-specific code. No it won't run as quickly as on the 5G, but that has nothing to do with not being designed to run on the iPod touch 4G. It will run faster on the iPhone 5 than on the iPod touch 5G - that is purely due to hardware specs, the same reason it will run faster on the iPt 5G than the 4G.
If you are familar with writing software, updating a software platform to include new features also allows the developers to revisit existing code and functionality and rewrite in order to improve performance. I have no doubt that that is what will have happened in this case. It does not automatically follow that a new release of iOS will run more slowly than the previous release. I would expect there to be performance improvements in some areas.