speedwatching.lua with toggle

local speedwatching = false

function onsub(name, text)
  if text == null or text == "" then
    mp.set_property_native("speed", 2.0)
  else
    mp.set_property_native("speed", 1.5)
  end
end

function toggle()
  if speedwatching then
    mp.unobserve_property(onsub)
    mp.set_property_native("speed", 1)
    speedwatching = false
  else
    mp.observe_property("sub-text", "string", onsub);
    speedwatching = true
  end
end

mp.add_key_binding("H", "speedwatching", toggle)