Applications that use browser engines for rendering tend to be a bit sluggy compared to native applications, yes. But I don't think a common complaint is that a web browser as a standalone application is particularly slow either running or starting up. People tend to say stuff gets slow once they have a ton of tabs open, which makes sense.

Don't know how common it is, after all, people are used to all the slow stuff out there, maybe they don't even complaint when it's less frequent actions like opening a browser. Though at least for a ton of tabs, there are hibernating solutions, so very annoyed people can at least find a workaround, unlike with the unfixable startup delay

They are? My Linux PC have horrible perf (a 2014 Core M, 4gb RAM), and yet, Firefox opens instantly.

And Windows? And Chrome?

tbh, it's been 10 years I don't use Chrome... When I used Windows, my main browser was Edge, and was quite fast to open as far I remember, but I believe on Windows Edge stays open in background all the time anyway...

Also, what's "instantly", how much time exactly on a cold/hot start does it take after you press a key and a fully drawn/functional window is shown? Checked on an old Windows desktop (hot start), and while the empty window is shown in 0.8 sec (not too slow, but still not instant, those would be some old native apps taking 4 times less time), the fully drawn/functional window takes 2-3 sec, so definitely nothing instant