It can be, because it is :) If you want a really technical explanation, you can profile the application. If that is too much work, I'll generalise and say that the tendency for the UI to lock when performing CPU heavy computations makes it feel slow. Because of the things you will be asking it to do (i.e. solve modern problems), sometimes such computations exist and lead to blocks of 1s or more. That will never feel fast.
Booting up is normally not done so often, but also doing other stuff takes way too long for my taste (over the perceivable 300ms threshold).
> Emacs ran on hardware from the 1980’s. How can it be too slow on modern hardware?
Since then they've added millions of lines of ELisp code. The comparison is moot.
Also we are talking here about Emacs "distributions" (Spacemacs, Doom Emacs) which come with a lot of code, and ELisp is not particularly fast.
I've just tried Doom Emacs on 27.1, with the default stuff plus some minor additions, and it starts within 900 ms on my laptop (Intel(R) Core(TM) i7-9850H CPU @ 2.60GHz).
HOWEVER, a heavily and naively reconfigured emacs, possibly with uncompiled elisp code and copy-pasted invocations that wait for network responses could take ages to open.
If you are finding emacs to be slow, try it vanilla first ("--no-init-file") before deciding it's the program and not your configuration.