Real world examples will be the best answers :)
One of the basic examples I can think of, is when the front-end developer builds the website not in an efficient way that causes some actions to be slow.
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IMHO, I don't think the question has one solution and also does not approaches the intent in the right way .
This is a classic delivery mindset problem . Isn't it mandatory for the Front end developer to actually make sure the User experience is the numero uno priority , right from utilizing cache effectively to making sure there is no complex loops in our JS .
It is understood that the strict timelines we'd have to adhere to sometimes causes things to roll out of control,but when we step back and do a usability testing ofvarious scenarios , for eg: network throttling. we can see a number of issues.
Fortunately , UX is not only relegated to a bunch of fancy animations and ARIA color contrasts anymore.
From a developer perceptive, it would be better if we take a look at these things
My two cents based on experience :) !
Irrespective of the department of website development, anything that makes the user experience of the site unpleasant directly affects on how the user reacts to your product.
The example you put above on the front-end development is just one entity of the website development team. Imagine that the front-end developers have done a great job in optimizing the website to the best, but the data coming from back-end is taking a lot of time. And when both (the front-end and back-end developers) have done the perfect job, but the user is not able to consume the content well, because the content developer has not written understandable content. And the list goes on....
Eventually, everyone in the team have to do their jobs well to make the user experience of the site usable, delightful, understandable; so the users use your product without getting annoyed.