Are there general or specific iOS6 performance issues on iPhone 4?
I have been running iOS 6 since Friday night (I ran the Golden Master that became release) on my iPhone 4.

Safari feels snappier (and show improvement on SunSpider benchmarks).

I have not noticed any degradation like the 3G iOS 4 craziness that apple gave us (I went through that as well). 

I am very happy and using it (everyday) and it feels a bit better in most things than 5.