How increase connectTimeout over 10s in PassThroughCluster or BlackHole OR increase Connection to Istio proxy

Describe the feature request
My service running on Istio and traffic from internet go into Nginx first, then proxy pass to istio ingress-gateway. So:
I was load test to my service running on Istio when reach 2000 request/s everything is OK. But when load test over 3000 request/s I see request 502 in kibana dashboard.
Everybody see this issue and optimize tcp connection created to Istio proxy ingress/sidecar proxy or increase ConnectTimeout 10s in PassThroughCluster or BlackHole.

I was increase tcp_max_connection, http_max_connection in DestinationRule but request 502 still in kibana dashboard.

Additional context
EKS: 1.14
Istio 1.3.3