What is the test for purposeful delay in bringing criminal charges?

"Under the California standard, 'negligent, as well as purposeful, delay in bringing charges may, when accompanied by a showing of prejudice, violate due process. . . .' '[W]hether the delay was negligent or purposeful is relevant to the balancing process. Purposeful delay to gain an advantage is totally unjustified, and [where purposeful delay is shown] a relatively weak showing of prejudice would suffice to tip the scales towards finding a due process violation. If the delay was merely negligent, a greater showing of prejudice would be required to establish a due process violation.' [Citation.] The justification for the delay is strong when there is 'investigative delay, nothing else.'" (People v. Cowan, supra, 50 Cal.4th at p. 431.)