well,
usually anything over 300 ms I would ask a detailed explanation from the back-end developer.. but that's just me.. explain plan on every single query that hits the database and the SAT tool with breakdowns in functions, etc - the whole thing.
sincerely, I don't like writing code that takes less than a millisecond to run inside a browser with limited resources, having to run jsperf on every single browser testing approaches because different js-engine would perform slightly different on a .forEach vs for ++ to see a Query in ABAP running a full table scan because some developer has no idea what an index is..
there must be some real good reason for anything to take three seconds, Google for instance returns in 400 ms and I'm sure they are querying a hell lot of data.
recent case, we needed to query over a field which is a text type that was holding an XML.. it has been used some "text type thing" in ABAP where the contents of XML was broken into lines of 255 and appended over - some BAPI code would bring the contents of that field.. that itself was taking 400 ms - my question was: "why the heck this XML is not just a clob or something? - why use a BAPI that takes this long when there are performance efficient ways to write the same solution?" - I ended up with the traditional "this is how we do in ABAP" sort of answer.. pretty sure you have experienced something along these lines so far.
UI5 somehow is maturing to a point of continuous integration with performance testing, coverage, unit testing, mocking and recently a dependency injection of modules using require to only find out in many cases that "the back-end" doesn't even know what a unit test really is.. so I find a bit bizarre to have this maturity on the UI code when the core business code lack the basics.
this is my personal opinion and there's no need for you to be offended.