Complex sentence: During the editing, Welles showed a rough cut to the visiting head of the Cannes Film Festival, who immediately wanted to include the film in the festival, requiring Welles to finish the editing at a faster pace than he preferred.
Split into simpler sentences: During the editing, Welles showed a rough cut to the visiting head of the Cannes Film Festival. The festival head immediately wanted to include the film in the festival, requiring that Welles finish the editing at a faster pace than he preferred.