Sixth Amendment to the United States Constitution
The right to a speedy trail originated with the Assize of Clarendon in 1166. It says that justice must be provided to robbers, murders, and thieves "speedily enough."  Those who wrote the Bill of Rights did not want defendants to be prisoners for a long time without the benefit of a trial. They did not want the accused to have their life disrupted, especially when the Fifth and Sixth Amendments both hold a presumption of innocence. Also, the longer a trial is delayed, the more likely that evidence may be lost and that witnesses may forget or cannot be found.