The work is largely the same.

You think about allocations: JS is a garbage collected language and allocations are "cheap" so extremely common. GC is powerful and in most JS engines quite fast but not omniscient and sometimes needs a hand. (Just like reasoning with any GC language.) Of course the easiest intervention to allocations is to remove allocations entirely; just because it is cheap to over-allocate, and the GC will mostly smooth out the flaws with such approaches, doesn't mean ignoring the memory complexity of the chosen algorithms. Most browser dev tools today have allocation profilers equal or better to their backend cousins.

You think about threading, concurrency, etc: JS is even a little easier than many backend languages because it is (almost excessively) single-threaded. A lot of concurrency issues cannot exist in current JS designs unless you add in explicit IPC channels to explicitly "named" other threads (Service Workers and Web Workers). On the flipside, JS is a little harder to reason about threading than many backend languages because it is extensively cooperatively threaded. Code has to yield to other code frequently and regularly. Shaving milliseconds off a routine yields more time to other things that need to happen (browser events, user input, etc). That starts to add up. JS encourages you to do things in short, tight "bursts" rather than long-running algorithms. Here again, most browser dev tools today have strong stack trace/flame chart profilers that equal or exceed backend cousins. Often in JS "tall" flames are fine but "wide" flames are things to avoid/try to improve. (That's a bit reversed from some backend languages where shallow is overall less overhead and long-running tasks are sometimes better amortized than lots of short ones.)

> But someone browsing my webpage one time ever? That might matter a lot less to me, you're not "browsing in a hot loop".

The heavily event-driven architecture of the browser often means that just sitting on a webpage is "browsing in a hot loop". Browsers have gotten better and better at sleeping inactive tabs and multi-threading tabs to not interfere with each other, but things are still a bit of a "tragedy of the commons" that the average performance of a website still directly and indirectly drags everyone else down. It might not matter to you that your webpage is slow because you only expect a user to visit it once, but you also aren't taking into account that is probably not the only website that user is browsing at that moment. Smart users do directly and indirectly notice when the bad performance of one webpage impacts their experiences of other web pages or crashes their browser. Depending on your business model and what the purpose of that webpage is for, that can be a bad impression that leads to things like lost sales/customers.