You can't assume from your limited experience with a 3G running iOS 4.2 that an iPhone 3GS running iOS 6 or an iPhone 4 running iOS 7 will be as bad.
Those issues were caused by the anemic processor and 128mb of RAM in the original 3G. The 3G was arguably unusable with iOS 4.2 but the same situation really hasn't repeated itself with more recent devices.
thank you. i did say precisely this, that the current fears are measurably unjustified, but obviously i didn't make clear enough that this was what i meant. :/
Those issues were caused by the anemic processor and 128mb of RAM in the original 3G. The 3G was arguably unusable with iOS 4.2 but the same situation really hasn't repeated itself with more recent devices.