prime_numbers.groovy

def get_atmos_list = { input ->
  def time_start = System.currentTimeMillis()
  def atoms = [2, 3]
  def fill_list = { n ->
    def sqrt = Math.sqrt(n) as int
    def is_atom = true
    for(i = 0; i <= sqrt - 1; i++) {
      if(n % atoms[i] == 0) {
        is_atom = false
        return
      }
    }
    if(is_atom) { atoms << n }
  }
  def next = 4
  while(next < input) {
    fill_list(next++)
  }
  def time_end = System.currentTimeMillis()
  def time_diff = (time_end - time_start)
  return [
    atoms: atoms,
    time: time_diff
  ]
}


//println get_atmos_list(10)
//println get_atmos_list(100)
//println get_atmos_list(1000)
//println get_atmos_list(10000)
println get_atmos_list(1_00_000)

Incluso en la versión WEB uso lo que comentas de n=+2 :P https://github.com/rodrigoSaladoAnaya/criba_eratostenes/blob/master/index.js#L26