When you're dealing with a site of that scale, it's probably best not to shift to Joomla. Joomla can handle a fair bit, but I'd suggest that it would be far easier to have a Joomla site that large when the site has been adapting as the traffic increases.

Part of what makes your question hard to answer is that we don't know much about what your current site does.

• Is it mainly a CMS? If so, Drupal may be a decent fit for you.

• Is it a custom web application? It sounds like it could be, based on the number of translations you're handling.

• Is it somewhere in-between? This could also be what you're dealing with.

If you're looking at either of the bottom two options, yes, shifting away from SilverStripe (but not to Joomla) sounds like it might be a good thing.

Given the volume of traffic you're dealing with, it sounds like you probably already have a revenue stream and a development team. In that case, it might be worth having a few of your developers play with CodeIgniter and Yii and see what they think.

I wrote up a detailed answer to a question about what type of framework to choose a while ago, which you can look over here:https://stackoverflow.com/questions/10095906/yii-or-zend-or-other-frameworks-under-the-following-requirements/10115678#10115678

I'm a Yii fan, having checked out a bunch of other frameworks. Regardless of which framework you use, I'd like to offer one piece of advice that I've found key on all projects I've worked on.

You want to work with objects, not data arrays

The reason for the above is that your models (and the objects they produce) can end up having your business logic built into them. Doing this makes life far, far easier in the long run. A great book on the subject, aimed at manager's and quite high-level (there is no code, although there might be a few lines of psuedo-code), is Object Technology: A Manager's Guide. I'd highly recommend it.

Framework thoughts

CakePHP

I started dabbling with CakePHP until I found out it didn't hand back objects (just arrays), at which point I ran screaming.

CodeIgniter

Looking at what CodeIgniter has for database model support, I think you would quickly hit its limitations with a site of your size, so would recommend against it. And having EllisLabs deciding to leave CodeIgniter behind (see their website for more details) is not a good sign. The signs say stay away.

Of the PHP frameworks out there, these are the ones I hear the most things about:

• Symfony 2
• Yii (disclosure: I work with this one daily)
• Laravel

Symfony

From what I can see of it, has been a huge stimulus to the PHP community and many best practices have come out of that community. Unit testing, Composer, pushes for PSR-1/2, the ideal of modularity of code (some of their core code is ending up in Drupal 8) are all great things that have spurred the PHP community into writing code that's better than spaghetti code. With that said, it feels kind of heavy and enterprise-y and looked to have a pretty heavy learning curve.

Yii

When I was looking for a framework, I had a lot of legacy code that needed migration. Yii's way of handling things, although opinionated at times, has done a lot for me as far as exposing me to good practices that are used heavily on the web. Highly recommended as well.

Laravel

When I was looking for a framework (2+ years ago), Laravel didn't exist. It's apparently quite slick and cutting edge (haven't looked at it yet, I've been busy landing new work regularly in the Yii world), but I am interested in dabbling with it. That said, here's a thoughtful article about Laravel and couple of considerations that I've wondered about.

1. The main developer, Taylor Ortwell, seemed to be a dominant force. That's a great thing while he's making money doing this and enjoying it. Not sure what would happen to the framework if he decided to do something else.

2. The speed of development on the framework is an awesome thing and means that all sorts of creative juices are flowing. It's a great place to go look at cool new things. That said, you're going to want your developers to be able to easily find information about the framework they're working with, which in 3-6 months may be severely out of date in the Laravel community. The Ruby on Rails community has the same type of good/bad problem when I've looked around over there; there is so much information about the new stuff that sometimes it's hard to find information about the version of the framework you're dealing with.

That said, there are lots of cool choices out there :-) I should probably wrap up, as this has gone on far longer than I'd intended. Let us know what you end up deciding and how it ends up going.

How big is the site really? Bigger than joomla.org?

Are you talking about big in terms of the amount of content in the database or in terms of the number of visitors.
Joomla really does not have problems with scale unless you are on an older version of MySQL in which case it slows down.

It depends on what kind of application you want.If you want a CMS, Joomla is the way to go. Because this is what it was build for. If you are about to create a real "custom" application Yii/Codeigniter is the way to go. (This is what yii was build for)

Regarding performance, from my expericnece Yii is a lot faster, because it comes with way less overhead. You can build the application just as you need it.