用node测试同时发起请求，同时发起1000个，请求时间远远单个最久的请求时间。按道理，最大耗时不应该是其中单个的最久耗时么？但好像并不是这么回事？求大佬解惑

const request = require('request');

function _request(url,data,callFun){
    request({
        url: url,
        method: "POST",
        json: true,
        headers: {"content-type": "application/json"},
        body: data,
    }, (error, response, body) => {
        callFun && callFun(error,response,body);
    });
}

function _send(start,len = 1000){
    let url = 'https://www.baidu.com';
    return new Promise((resolve,reject) => {
        let max = start + len, 
            sendResult = {success:0,fail:0};
        for(let _i = start; _i <= max; _i++){
            let data = {_i};
            _request(url,data,function(err,res,body){
                let _r = Math.ceil(Math.random()*2) - 1;
                if(_r){
                    sendResult.success++;
                }else{
                    sendResult.fail++;
                }
                if(_i === max){
                    resolve(sendResult);
                }
            });
        }
    });
}

async function test(){
    let sT = new Date().getTime();
    let userLen = 5000,
        limit = 1000;
        sp  = Math.ceil(userLen / limit);
    
    for(let i = 0; i < sp; i++ ){
        let _sT = new Date().getTime();
        let _start = i*limit;
        let result = await _send(_start,limit);
        let _eT = new Date().getTime();
        console.log(result);
        console.log('本轮执行完毕,耗时:' + (_eT - _sT));
    }

    let eT = new Date().getTime();
    console.log('全部执行完毕,共耗时:' + (eT - sT));
}

test();