For example if the the app is too slow because we are pulling in thousands of records and rendering it to the DOM, the correct optimization is to paginate the results. Both frontend and backend devs should know that. However the frontend dev needs to make the pagination a fluid UX for the user, decide between page controls or infinite scroll, implement search on type, set the correct aria-attributes so assistive technology knows this is an incomplete list, etc. The backend dev however needs to optimize the database queries, decide on the type of pagination (keyset vs. limit-offset vs. cursor etc.) and map the response such that the frontend won’t have to do too much work ones received (I guess a frontend dev should be able to do the latter; but it is hard to ask a frontend dev to be an expert in optimizing database queries; or implement proper keyset pagination [although if the UX team decides on infinite scroll that limits the possible pagination types a little]).

> If you go even 7 years back you’d have needed to consider all these as a “back-end” developer

I have been a frontend dev for almost 15 years, and I very much disagree with you here. I don‘t think that in 2018 you could expect a backend dev to implement responsive layout, deliver accessibility, localization, delivering to different browsers (case in point: Intl.PluralRules became widely available 7 years ago).

This may have been true in 2009 when web-apps were fairly simple (often just a series of form submissions), you had a good idea about the screen size of your users, it was hard to mess up accessibility (and yet, many developers did), localization was mostly done on the backend, single page apps were almost unheard of, etc. But not in 2018.

This is all HTML & CSS. Bootstrap made responsiveness normal ever since it was introduced. If you have any form of server rendered application you have to handle these. It’s not like you can choose to hand it to a nonexistent front-end dev, since there is no separate front-end.