Instant Forensics

In real life forensics labs have a backlog and several tests take time to actually process. In fiction land DNA tests, fingerprint analysis, and other forensic tests take, at most, the time it takes to get the evidence to "the lab".

Comic Books

 * Justified when Barry Allen, a forensic scientist, is The Flash.

Live-Action Television

 * Averted, though generally still unrealistically quick, in Law and Order where investigations take place over a longer period of time. The detectives will continue working their case until they are told latter the results of whatever test are in. The franchise does play it alarmingly straight when evidence to be tested is introduced midstory.