ExpressJS - redirect after send view

fairly new to Express, im trying to render simple ejs loading view, then do some asynchronized code, then load other view/redirect to other route and deal with the other view there. (thinking about the second option because i cannot change sended data after its been sent) im getting [ERR_HTTP_HEADERS_SENT] for the redirect line, can i get some help with the situation?

const app = express();
const port = process.env.port || 2000
const puppeteer = require('puppeteer');
const fs = require('fs')

app.set('view engine', 'ejs');

app.get('/', async (req, res) => {
    res.setHeader('Content-Type', 'text/html')
    res.send(fs.readFileSync('./views/loading.ejs'))
    if (fs.existsSync('./cookies.json')) {

        //some async 10 sec code (puppeteer)
    }
    else{ 

        //some async 10 sec code (puppeteer)
    }
    res.redirect(301,'/items')
})

app.get('/items', async (req, res) => {
    res.setHeader('Content-Type', 'text/html')
    res.send(fs.readFileSync('./views/items.ejs'))
})


app.listen(port)