How to warm envoyProxy before accepting hundred thousands websocket connections?

When I start a lot of connections to EnvoyProxy, I see some latency on some clients, and after a couple of minutes, it's gone. So I think I can do something like "warming up" before allowing clients to connect.

I want to set up some parameters:
- initial_stream_window_size
- initial_connection_window_size
- preconnect_policy
- per_connection_buffer_limit_bytes

Which k8s kind should I use for tweaking those parameters? EnvoyFilter?