Disabling SSL compression. This saves about 300K per SSL connection.
The SSL_OP_NO_COMPRESSION option is available since OpenSSL 1.0.0.

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 259b1d8..aa9a4c9 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -175,6 +175,10 @@
         SSL_CTX_set_options(ssl->ctx, ngx_ssl_protocols[protocols >> 1]);
     }
 
+#ifdef SSL_OP_NO_COMPRESSION
+    SSL_CTX_set_options(ssl->ctx, SSL_OP_NO_COMPRESSION);
+#endif
+
     SSL_CTX_set_read_ahead(ssl->ctx, 1);
 
     SSL_CTX_set_info_callback(ssl->ctx, ngx_ssl_info_callback);