Linux also opens apps instantly, at least for me and if you ignore that some app after being opened instantly don't get instantly ready to work.
But that isn't fault of the OS.
Through some OSes used some tricks to hide this loading time.
A good example is cold starting a web browser. Modern web browsers have to handle so much that just loading the amount of code they have all at once can lead to a noticeable delay. I mean e.g. the network code your browser runs is likely a few hounded to thousand times more complicated then what you put into a simple HTTP server. There is also the rule of thumb that the difference between something working in your use-case and it working in as many situations as possible for as many people as possible might look small from an external POV but in general comes with an explosion in complexity and code. And for browsers that is not just the case for networking but also rendering HTML+CSS, executing JS, storage handling, window handling, handling boundaries and security, extensions, caching, input handling etc.