Besides what others have already mentioned, third party devs (including myself) find ourselves disabling features on the iPhone 4 also.
The important thing to recall is that the iPhone 4 quadrupled the number of pixels you had to render while doing almost nothing for CPU performance. Graphics performance-wise the iPhone 4 was a huge downgrade from the 3GS.
If you're doing any sort of graphics work that's CPU-bound, your iPhone 4 performance is going to be dismal.
The important thing to recall is that the iPhone 4 quadrupled the number of pixels you had to render while doing almost nothing for CPU performance. Graphics performance-wise the iPhone 4 was a huge downgrade from the 3GS.
If you're doing any sort of graphics work that's CPU-bound, your iPhone 4 performance is going to be dismal.