Nginx upstream keepalive connections improve load balancer performance

Without keepalive connections to upstream Express servers, Nginx creates new TCP connections for each request, adding latency and overhead. Connection establishment overhead accumulates under high request rates.

Configure Nginx upstream block with keepalive 32 to maintain persistent connections to backend servers. Set proxy_http_version 1.1 and appropriate proxy_set_header directives for Connection upgrade, Host, X-Real-IP, X-Forwarded-For, and X-Forwarded-Proto headers. Use proxy_cache_bypass for upgrade requests.