slow.go

package main

import (
	"flag"
	"os"
	"time"
)

func main() {
	var n int
	var s time.Duration
	flag.IntVar(&n, "n", 100, "bytes")
	flag.DurationVar(&s, "s", 100, "sleep")
	b := make([]byte, n)
	for {
		time.Sleep(s * time.Millisecond)
		os.Stdin.Read(b)
	}
}

slow.vim

function! s:test() abort
  let job = job_start(['slow', "-n", "500", "-s", "1ms"], {'noblock': 1, 'mode': 'raw' })
  let ch = job_getchannel(job)
  let n = 0
  while 1
    call ch_sendraw(ch, '0123456789')
    redraw
    let n += 10
    echo n
  endwhile
endfunction

call s:test()