WordPress digitalocean taking forever to load

So, I have a WordPress droplet in Digitalocean with 4 shared vcpu and 8gb of ram. Everything was working really fine until I get about 400 simultaneous access in the website, then it gets REALLY SLOW. But my ram and cpu are like 40%.

Now that I’m posting this question, that there’s 20, 40 people online it goes well. But when I have about 400 people, the website loads really slow – but memory and CPU using 40%.

Does anyone knows what could it be and how can I solve it?

Can it be a problem with my mysql settings? Or anything else?