A quick comparison how fast images load on the iPhone using a simple UIImage imageWithContentsOfFile:. Each image is 8-Bit RGB without Alpha:
| Filename | Bytes | msec |
|---|---|---|
| img-10000×50.png | 224193 | 283 |
| img-50×10000.png | 200397 | 643 |
| img-900×600.png | 879483 | 623 |
| img-30000×100.png | 727709 | 1222 |
| img-150×20000.png | 911923 | 2218 |
Looks like the horizontal strip is the winner when it comes to loading speed.