Depends.
Yes, out of the box, D7 is slower than D6. But a lot of effort has been put into making it scale (Which is a different thing than being performant).
Many of these features are actually already part of Pressflow 6 like support for master/slave database cluster, proxy, improvements to alias handling and so on. So that doesn't help you much.
But there are also features which did not exist there, for example the pluggable field storage that allows you to store all your fields in MongoDB for example. Also, while D7 in general is slower, the queries have been improved a lot and the bottleneck basically moved from the database to PHP, which is easier to cluster.
After all, D7 is able to power sites like http://examiner.com :) And they explicitly chose D7 over D6 for scalability improvements like MongoDB integration.