simplified version of https://gist.github.com/tompng/2ab7f6a7c5fd5d7846fcece86b30a156

terminal_slow_motion.rb

require 'io/console'
require 'pty'

command = ARGV.join(' ')
if command.empty?
  puts 'Error: Command not specified.'
  puts "Usage: `ruby #{File.basename __FILE__} options -- command`"
  exit
end

class SequenceReader
  def initialize(input)
    @input = input
    @buf = nil
  end

  def read
    loop do
      c = @input.getc
      return unless c
      if @buf
        @buf << c
        if @buf.match?(/\A\e([^\[O]|\[[?;\d]*)[^?;\d]|O./)
          buf = @buf
          @buf = nil
          return buf
        end
      elsif c == "\e"
        @buf = c
      else
        return c
      end
    end
  end
end

wait = 0
wait_write = ->(a = 1) { sleep a * 0.001 * 2**(wait - 1) if wait != 0 }

puts 'F1 : Rendering speed += 1'
puts 'F2 : Rendering speed -= 1'

PTY.spawn command do |pty_input, pty_output|
  update_winsize = -> { pty_output.winsize = $stdin.winsize }
  Signal.trap(:WINCH, &update_winsize)
  update_winsize.call

  Thread.new do
    $stdin.raw do
      stdin = SequenceReader.new($stdin)
      loop do
        data = stdin.read
        case data
        when nil
          exit
        when "\eOP" # F1
          wait = [wait - 1, 0].max
        when "\eOQ" # F2
          wait = [wait + 1, 10].min
        else
          pty_output.write data
        end
      end
    end
  end
  reader = SequenceReader.new pty_input
  loop do
    c = reader.read
    exit unless c
    case c
    when "\e[?25l"
      # ignore cursor hide
      next
    when /\A\e\[.+[JKLM]/
      # use default color for clear line or screen
      wait_write.call 10
    when /\A\e\[.+[A-ISTZ]/, /\A\e[DEM]/, /[\x0A-\x0D]/
      wait_write.call 10
    end
    wait_write.call
    $stdout << c
  end
end