SSL Termination Performance with Nginx on EC2

I'm running an nginx instance that's acting as an SSL termination point and basic load balancer on an EC2 virtual server, and seeing very poor performance for the SSL pages served from the upstream source.

The EC2 instance is a c1.medium, and ought to be able to sustain a reasonable throughput, but I can't get it above 60 transactions per second.

Serving the nginx status page directly off the server, I manage more than ten times the throughput, so it's not purely SSL overhead, but if I reconfigure it to serve the same content without SSL I also do very much better, so it's also not upstream overhead. The CPU is maxed out while it's serving its 60 transactions per second.

I'm using ab to test it, with parameters "-n 1000 -c 50 -k" -- 1000 hits, concurrency of 50, keepalives enabled so that the SSL session caching ought to work.

Here's an abbreviated config:

user www-data;
worker_processes 4;
pid /var/run/nginx.pid;

events {
    worker_connections 4096;
}

http {
    sendfile off;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 65;
    types_hash_max_size 2048;

    include /etc/nginx/mime.types;
    default_type application/json;

    log_format standard '$remote_addr - $remote_user [$time_local] "$request" $status $body_bytes_sent "$http_referer" "$http_user_agent"';

    access_log /var/log/nginx/access.log standard;
    error_log /var/log/nginx/error.log;

    gzip on;
    gzip_types text/plain application/json;
    gzip_comp_level 1;

    upstream test {
    server 10.226.31.66;
    }

server {
    listen   443;

    ssl on;
    ssl_certificate     /etc/nginx/certs/both.crt;
    ssl_certificate_key /etc/nginx/certs/https.key;

    ssl_session_timeout  10m;
    ssl_session_cache   shared:SSL:10m;

    ssl_protocols  SSLv2 SSLv3 TLSv1;
    ssl_ciphers  ALL:!kEDH:!ADH:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP;
    ssl_prefer_server_ciphers   on;

    location /v1/system/nginx {
        stub_status on;
        allow all;
    }

    location /nginx_status {
        stub_status on;
        allow all;
    }

    location / {

        proxy_pass      http://test;
        proxy_set_header   Host $host;
        proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;

        client_max_body_size   10m;
        client_body_buffer_size 128k;

        proxy_buffering on;
        proxy_connect_timeout 15;
        proxy_intercept_errors on;
    }
    }
    }