In real life forensics labs have a backlog and several tests take time to actually process. In fiction land, though, DNA tests, fingerprint analysis, and other forensic tests take, at most, the time it takes to get the evidence to "the lab". These results often require a Magical Database to get their match. In works set in the future, this may be justified through a gadget, possibly an Everything Sensor and/or Super Identikit, that performs the tests without the need for a human to.
In the real world, this has resulted in juries being more resistant to non-forensic evidence and easily swayed by "evidence" of questionable scientific validity. This phenomenon is known as The CSI Effect.
As an Omnipresent Trope only exceptions and non-straight examples will be listed.
- Justified when Barry Allen, a forensic scientist, is The Flash.
- 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.
- Played with in Murdoch Mysteries, where the detective will often have to invent the forensic science needed to solve the crime. Often Historical Domain Characters of the early 1900s will help Murdoch to invent the appropriate devices, like Nikola Tesla. In one early episode, Murdoch has a beautiful steampunk Lie Detector -- which the writers soon realized that they could never bring back as it would be a total Game Breaker.
- Similarly played with in Hec Ramsey, where the forensics tech is so primitive it can only return immediate results, because it can't do anything sophisticated that would take a long time. Here, it takes longer to collect the evidence than it does to analyze it.