Author here, nice to see this got some interest! As a very small benchmark (uh-oh!), in a console on the linked page, I tried to make/recreate 10,000 divs:

  const root = document.getElementById('noughts')
  m.render(root, {children: [...Array(10000)].map(_=>m('', {class: ['hi']}, 'hi'))})
  m.render(root, {children: [...Array(10000)].map(_=>m('', {class: ['bye']}, 'bye'))})

wrapped with:

  console.time('a');  ... console.timeEnd('a')

On my laptop (admittedly a fairly new macbook), I got (approx):

  130ms to make divs from scratch
  80ms to switch from 'hi'->'bye'
  50ms to re-render with no changes

I'd be super interested as to what comparable figures would be for React. There's a fair bit of interesting discussion below surrounding performance, but with no datapoints.