you can always optimize WP in many ways:
- install caching plugin (w3 total cache)
- use CDN to serve all your static content (images, js, css)
- optimize server better (mysq, apache, xcache/eaccelerator...)
so it's a good platform to make even big sites, but you'll have to do more to optimize it...
the biggest problem can be MySQL but caching plugins can take care of it.
If you need any help with WP optimization or some consulting if you're planing to build a large site based on WP - don't hesitate to contact me:
http://www.awmzone.com/services