Iphone – How many halfscreen images can I add to an animated UIImageView before performance would start to suck

In general, a movie will have applied some compression and possibly even used a lossy compression. This means the processor would need to work harder but it has a lot less memory to read. The CPU is a fast resource. Compared to the CPU, memory is slow. Thus a compressed movie would (logically) have the better performance.

In practice, it could depend on a lot of factors, although movies do tend to be better optimized for animations. With a slow CPU and extremely fast memory, a multi-image animation might just be faster. Also, it depends on how you store those many images. But in 99% of all situations, movies will have better performance.

You are right about imageNamed... you should not use it in many cases as it internally caches the image. Use:

[UIImage imageWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"image1" ofType:@"png"];

The only time you want to use imageNamed is when

1) You are reusing an icon over and over (tableviews etc...)
2) You have not implemented your own caching system