A hand plane forces a different pace than a router does. There's no way to rush it without the wood telling you immediately that you've rushed it — a plane that's pushed too fast just judders and tears rather than producing a shaving, in a way that's impossible to ignore or work around.
The slowness isn't a limitation of the tool. It's the entire point of using it.
I don't use hand tools for everything — but for the parts that matter, I've stopped reaching for the power tool first. A router will flatten a board faster than a hand plane ever will, but it won't teach you anything about the grain direction changing halfway across the board the way a plane immediately will, the moment it starts tearing rather than shaving cleanly.
There's a temptation to treat this as nostalgia, or as making things harder than they need to be for the sake of it, and some days it genuinely is just slower with nothing gained. But often enough, the slower process surfaces a problem with the material or the joint that the faster tool would have hidden until it was too late to fix cheaply.
It's not a rule I apply everywhere in the workshop. It's a preference for the specific moments where getting it right matters more than getting it done, and where the tool itself is quietly telling you something useful if you're paying attention.