if a customer is drinking a single espresso shot, the flavour does change over time. you can see a shot “die” as the light crema fades away leaving a dark black shot.

a proper, well made shot of espresso should last long enough for someone to drink it. but starbucks beans are over roasted, less fresh, made with too much water, and are made with automatic machines. all of these things lead to a shot that goes bitter more quickly.

speaking of drinks with milk- lattes, etc: the shots do not taste different when mixed. starbucks is lying to you. they say this to make you move taster. but the quicker our bitter shots are neutralized by milk, the better. stops the espresso from oxidizing and becoming even more bitter.

It's complicated. Taste-wise there might be some change as elements break down (less roundness, less boldness, possibly some bitter or astringent elements), but with espresso, you get out what you put in. For all of the criticism over how StArBuckS RoAstS ToO DaRK, that dark roast serves its purpose in terms of consistent taste across a large quantity of beans and it is pretty much the best option when we are talking economies of scale. It will just taste the way it tastes, like it or hate it.

The main issue is how the shots incorporate with milk. Shots are going to break down chemically and crema is also going to diminish (already low in a shot at Starbucks with less freshness of beans/dark roasts). These breakdowns affect how well the shots mix with the milk. Shots that have sat out a bit longer are going to dominate a bit more flavor-wise in say, a latte, as the chemicals in espresso that would typically bond with fats and other lipids in the milk are among the first to break down.

Is your latte going to taste disgusting? Possibly, also possibly not depending on if the shot itself is tasty and if you like the taste of espresso (the drink likely will be more espresso-forward).

Are you going to have more difficulty doing things like latte art or crafting that "perfect" latte? Yeah, probably.

All in all, if you are sequencing two drinks at a time, it's probably a moot point since you will be able to incorporate well before the chemical breakdown becomes an issue.

Also $1 was just posted on YouTube by a fairly respected roaster/YouTuber.

Someone has explained this really well already I just wanted to add that shots of espresso should have 3 layers (I don’t think Starbucks shots really do because of the automatic machines and the way the shots are pulled, however, shots from an actually espresso machine have three distinct layers). And each layer adds a different flavor profile to the shot. The heart (bottom) is the bitterness, the body (middle) is the strong coffee flavor, the crema (top) is the sweetness. When you let the shot sit for a long time and it dies, the layers dissipate and it just becomes bitter and burnt tasting instead of balanced sweetness and bitterness.