The question--why is the tempo of a fight that occurs within a play important--does not specify a particular play. There have, however, been plenty of examples throughout the history of theater in which fight or battle scenes have been incorporated into the action. Plays that were produced during Medieval times, or that were authored by Shakespeare, or that were produced in ancient Rome or during the present have included scenes of conflict, and these scenes have been structured for maximum effect through the use of, among other techniques, pacing. The pacing or tempo at which the fight is staged is manipulated for the purpose of maximizing the scene's dramatic impact or, conversely, moving the scene along as expeditiously as possible so that more important scenes or themes can be emphasized. It all depends upon the role of the fight in the broader context of the act or scene in which it occurs. Sword fights, which featured prominently in many plays during earlier periods, including adaptations of Alexander Dumas' The Three Musketeers and Count of Monte Christo, were staged in real-time tempos because of their centrality to the scenes in which they occurred. Fights staged in productions adapted from the works of Shakespeare will similarly use a tempo consistent with the rest of the action presented. Larger battle sequences, such as occur in Henry V, are adjusted for the obvious limitations of the stage (i.e., physical limitations in terms of the size of the stage and the number of actors involved), the fighting being depicted in truncated terms both because of those inherent limitations and because of the peripheral role of the actual fighting to the broader action.
Just as with film, fight scenes in plays are either sped up or slowed down depending upon the writer and/or director's vision for the role of the conflict in the broader story. The assassination of Julius Caesar is, obviously, a vital scene in Shakespeare's play, but, as readers and viewers of the play know, it is not central to the political machinations that provide the basis of the story the playwright wanted to relate. The scene, however, can be slowed down to emphasize its inherent brutality and the theme of betrayal involved, or it can be staged in normal pacing to emphasize its inevitability. Tempo is a tool directors use to manipulate audience sentiments and reactions.