Chances are that it might still not be as fast as Safari, but Firefox should bite the bullet and just do it because:
a. Firefox cannot afford to not be on all platforms. With computing devices converging and relying on handshakes (data syncing?), missing out on a major mobile platform is foolish.
b. In a related point, syncing bookmarks/tabs via Firefox sync is severely handicapped by it's absence on iOS.
c. There is a fantastic niche where FF can fit in on iOS. It'd be as privacy centric as Safari and with an iOS launch would be cross-platform like Chrome (without Google peddling their products front and centre). They could also get some inspiration from mobile Opera on iOS that has fantastic data saving features in-built.