Caver로 getBalance()어쩔땐 되고 어쩔땐 안되요

한국어🇰🇷 Caver
1 
import {createRequire}from "module";
const require=createRequire(import.meta.url);
const Contract =require("./build/contracts/Count.json");

import Caver from "caver-js";
const rpcURL="https://public-node-api.klaytnapi.com/v1/cypress";
const networkID="8217";
const caver=new Caver(rpcURL);

const address="0x7bf205526df859c7ceadbcca9af3c6f9ec74fbad";
const privateKey="0x1111111111111111111111111111111111111111111111111111";

async function test(){
    let ret;
    ret=caver.klay.accounts.createWithAccountKey(address,privateKey);
    ret=caver.klay.accounts.wallet.add(ret);
    ret=caver.klay.accounts.wallet.getAccount(0);
    console.log("getAccount:",ret.address);

    const deployedNetworkAddress=Contract.networks[networkID].address;
    const contract=new caver.klay.Contract(Contract.abi,deployedNetworkAddress);
    
    for(let i=0;i<99;i++){
        ret=await caver.rpc.klay.getBalance(address);
        ret=caver.utils.fromPeb(ret.toString(),"KLAY");
        console.log("balanceOf:",ret);
    }
}
test();

C:\Users\LG\Desktop\Minting>node mainnet.mjs
getAccount: 0x7bf205526df859c7ceadbcca9af3c6f9ec74fbad
balanceOf: 4.808382975
balanceOf: 4.808382975
balanceOf: 4.808382975
balanceOf: 4.808382975
balanceOf: 4.808382975
C:\Users\LG\Desktop\Minting\node_modules\caver-js\packages\caver-core-helpers\src\errors.js:92
        return new Error(message)
               ^

Error: Invalid JSON RPC response: {"code":1200503,"message":"error from node API; ","requestId":"9e22f802-8cc1-90fb-a1ce-2370a9d21338"}
    at Object.InvalidResponse (C:\Users\LG\Desktop\Minting\node_modules\caver-js\packages\caver-core-helpers\src\errors.js:92:16)
    at C:\Users\LG\Desktop\Minting\node_modules\caver-js\packages\caver-core-requestmanager\src\index.js:159:44
    at XMLHttpRequest.request.onreadystatechange (C:\Users\LG\Desktop\Minting\node_modules\caver-js\packages\caver-core-requestmanager\caver-providers-http\src\index.js:119:13)
    at XMLHttpRequestEventTarget.dispatchEvent (C:\Users\LG\Desktop\Minting\node_modules\xhr2-cookies\dist\xml-http-request-event-target.js:34:22)
    at XMLHttpRequest._setReadyState (C:\Users\LG\Desktop\Minting\node_modules\xhr2-cookies\dist\xml-http-request.js:208:14)
    at XMLHttpRequest._onHttpResponseEnd (C:\Users\LG\Desktop\Minting\node_modules\xhr2-cookies\dist\xml-http-request.js:318:14)
    at IncomingMessage.<anonymous> (C:\Users\LG\Desktop\Minting\node_modules\xhr2-cookies\dist\xml-http-request.js:289:61)
    at IncomingMessage.emit (node:events:538:35)
    at endReadableNT (node:internal/streams/readable:1342:12)
    at processTicksAndRejections (node:internal/process/task_queues:83:21)

Node.js v17.5.0

위쪽이 제가 작성한 코드, 아래쪽이 발생한 오류입니다. 이런 오류랑 더불어서 Error: Invalid JSON RPC response: No Healthy Upstream 이 엄청 뜹니다.
또한 네트워크 문제인지는 모르겠는데 getBalance 반환이 거의 3~5초씩 걸리는 등 굉장히 느려요. 원인이 뭔가요?