为什么 flink wordcount 程序跑了 10s？

import org.apache.flink.api.common.functions.FlatMapFunction;
import org.apache.flink.api.java.DataSet;
import org.apache.flink.api.java.ExecutionEnvironment;
import org.apache.flink.api.java.tuple.Tuple2;
import org.apache.flink.util.Collector;

import java.util.List;

public class WordCount {
public static void main(String[] args) throws Exception{
ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
long st = System.currentTimeMillis();
DataSet text = env.fromElements(
"Using the Hello World guide",
" you’ll start a branch",
" write comments",
"and open a pull request. ");
System.out.println("init" + (System.currentTimeMillis() - st));

long st2 = System.currentTimeMillis();
DataSet<Tuple2<String, Integer>> sum = text.flatMap(new FlatMapFunction<String, Tuple2<String, Integer>>() {
public void flatMap(String value, Collector<Tuple2<String, Integer>> collectors) {
String[] tokens = value.toLowerCase().split("\W+");
for (String token : tokens) {
if (token.length() > 0) {
collectors.collect(new Tuple2<String, Integer>(token, 1));
}
}
}
}).groupBy(0).sum(1);
System.out.println("handle" + (System.currentTimeMillis() - st2));

long st3 = System.currentTimeMillis();
List<Tuple2<String, Integer>> collect = sum.collect();
sum.print();
System.out.println("print" + (System.currentTimeMillis() - st3));

}

}