Ranger does not have a build-in keyboard macro mechanism, but you can use tmux for this.

Create the tm-send-slow command

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#!/bin/bash
export TTY

string="$1"

grep -o . <<< "$string" | while IFS=$'\n' read -r char; do
    tmux send -l "$char"
done

Create the mapping

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map F. shell -f tm-send-slow FFf

Demonstration

asciinema recording