As you most likely know, quest mode has you completing objectives in each stage. Depending on which faction you're playing as (Vigilantes, Coyotes, Drifters), you'll have slightly different objectives. However only the collectable objectives affect time spent on a stage.

This guide focuses on how the collectable objectives work and how they can affect runs (because the game doesn't explain this to the player). There are 2 kinds of collectable item objectives:

a. Collect x amount of items https://imgur.com/a/6voip

Killing all enemies will trigger a roughly 30 second countdown. I assume this was implemented to give the player time to find the collectables needed. If you have the required amount of collectables on you before killing the last enemy, the stage ends there. However if you don't have all required collectables you have two options, you can either wait out the countdown, or you can end the countdown earlier if you find all required items. A good example of this would be Utah for Coyotes, all 4 briefcases are located in a small area, and it's very easy to collect them all after killing whoever's fighting you there, which is faster than waiting out the timer there. Usually it's better to wait the timer off but always try to find them after clearing the map to save a bit of time.

b. Collect and deliver x amount of items https://imgur.com/a/jxQQb

Collect and Deliver objectives function very differently though. They also lenghten the time spent in a stage but it happens under different conditions. Whereas the first type of objective gives you an extra 30 seconds to collect all required items, in these kinds of objectives the stage WILL end as soon as you kill all enemies IF you don't have any of the collectables items on you. If you have at least 1 of the required items with you when you kill all enemies the game will give you extra time to deliver them to the specified location, otherwise the stage will just end. So for speedrun purposes it is highly recommended to completely ignore collectables if they require delivering, as it ends the stage sooner. If for whatever reason you grab one (or more) of the items deliver them ASAP, that way the stage will still end once all enemies are dead.

Hopefully this clears up any confusion with stages "hanging" for too long after killing all enemies and helps you save time on your runs.