Load testing serverless stack using Gatling

Hi,
I'm doing some load testing on my serverless app and I see that it is unable to handle some higher loads. I'm using API Gateway. Lambda(Java 8) and DynamoDB. The code that I'm using is the same as this from this link.
In my load testing, I'm using Gatling. The load that I configured is that I'm doing a request with 120 users, then in one minute I ramp users from 120 to 400, and then for 2 minutes I'm making requests with 400 constant users per second. The problem is that my stack is unable to handle 400 users per second. Is it normal? I thought that serverless will scale nicely and will work like a charm.

Here is my Gatling simulation code:

public class OneEndpointSimulation extends Simulation {
    
    HttpProtocolBuilder httpProtocol = http
            .baseUrl("url") // Here is the root for all relative URLs
            .acceptHeader("text/html,application/xhtml+xml,application/json,application/xml;q=0.9,*/*;q=0.8") // Here are the common headers
            .acceptEncodingHeader("gzip, deflate")
            .acceptLanguageHeader("en-US,en;q=0.5")
            .userAgentHeader("Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:16.0) Gecko/20100101 Firefox/16.0");
    
    ScenarioBuilder scn = scenario("Scenario 1 Workload 2")
            .exec(http("Get all activities")
                    .get("/activitiesv2")).pause(1);
    
    {
        setUp(scn.injectOpen(atOnceUsers(120),
                        rampUsersPerSec(120).to(400).during(60),
                        constantUsersPerSec(400).during(Duration.ofMinutes(2))
                ).protocols(httpProtocol)
        );
    }
    
}

Here are the Gatling report results:
Image link
I'm also receiving an error:
**i.n.h.s.SslHandshakeTimeoutException: handshake timed out after 10000ms ** -> This is usually approx 50 requests. It is happening when Gatling is starting to inject 400 constant users per second.
I'm wondering what could be wrong.
It is too much for API Gateway, Lambda and DynamoDB?