Uggh. Rebuild it already. Its only a few actions. It wasn’t built for this kind of app.
Python, C, Perl, whatever.
You sure it’s not the other way around? Did you see this? http://www.avnetlabs.com/php/php-framework-comparison-benchmarks
Check out CodeIgniter’s benchmarks at the very bottom when using various ORM / code cache.
Rails is very fast by default in my experience even under moderate high load but if you need to get killer performance you simply break out of ActiveRecord and use plain SQL and turn on caching.
In Twitter’s case they have extreme activity on a simple app. I’d load balance, memcached, and if that didn’t work slap C in front of it.