The keyboard pops up, but is not responsive right away. This very thing happened today on my work phone (which has a bunch of MS defender/enterprise policies and apps managed by the company, forced background app updates etc, which could explain this) but I recall it also being a regular thing with all Samsung flagships I've had over the years. It's the feeling of a very slight delay (it could be a matter of as low as 200 ms) for important components which are operating-system controlled, such as the on-screen keyboard. It feels laggy, as if an app or a background process impacts the responsiveness of low level OS features.

On iOS, this was never something I had to experience. Slow apps are killed, iOS is brutal in this regard, but it protects the core OS-level components such as the keyboard. Try it out, load a few apps and try switching between them, where one of them had the keyboard uplled up. This is something regular users will likely never feel, but if you've been around since amiga 500, you'll definitelly feel it.