The network is very busy right now, resulting in higher gas prices. If you use a gas price that is too low, your transaction will take a lot longer to confirm. I recommend that you use ETH Gas Station to see what gas price you should use.
You can read more about gas in general here: https://support.mycrypto.com/general-knowledge/ethereum-blockchain/what-is-...
You can do this way :
const EthereumTx = require('ethereumjs-tx').Transaction;
import Common from 'ethereumjs-common';
const customCommon = Common.forCustomChain(