The Speedy Trial Act of 74' states that a trial shall not be delayed more than 100 days, the Sixth Amendment states that you have the right to a speedy trial, they are basically the same thing the 74' Speedy Trial Act just puts a number to the Amendment.
The Criminal Justice system does a terrible job at providing speedy trials, some suspects spend years waiting for their trial, and get partial or biased jury members. The Criminal Justice system in general needs reformed but especially when it comes to speedy trials.