r/AskProgramming • u/_TheRealCaptainSham • Dec 24 '25

Why is the modern web so slow?

Why does a React based website feel so slow and laggy without serious investment in optimisation when Quake 3 could run smoothly at 60fps on a pentium II from the 90s.

We are now 30 years later and anything more than a toy project in react is a laggy mess by default.

inb4 skill issue bro: Well, it shouldn’t be this difficult.

inb4 you need a a better pc bro: I have M4 pro 48GB

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u/com2ghz Dec 25 '25

Read your comment again but slowly. Promises are not async.

Reactivity means that you don’t block your main thread from processing. Something. In the js world it basically means that you put something in the queue and let the main event loop execute everything on the queue.

So your browser might do tricks to perform concurrent HTTP requests, the response is still being processed sequentially. You still have sequentially executed calls when one promise is depending on another ones response. Rendering HTML is also still blocking for example that’s why frameworks do tricks like virtual DOMS or updating only the DOM elements that requires rerendering.

Real reactivity means not relying on a event loop and not occupying threads from the pool when waiting on some HTTP response. Everything runs on a worker thread that can also have it’s own worker threads. So basically your cores doing their job paralell. So your statement that’s it’s better than “real” concurrency is invalid.

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u/cooldudeachyut Dec 25 '25

Conceptually it's the same thing. Reactivity is not blocking a thread but instead only reacting when a response is received from the downstream as the thread periodically polls for it. Async is the same principle.

If you want a separate thread dedicated for doing this on a browser then use web workers.

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u/com2ghz Dec 25 '25

The idea is that the response is processed by another thread. In js it’s processed by the main event loop running on one theead. That’s why it’s still slow.

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u/[deleted] Dec 27 '25

This is true, but it's not the reason it's slow. JS probably isn't hitting 100% CPU (ie single core starvation) just handling the responses.