According to this blog post , it is estimated that there will be internal runtime improvements that should help out of the box.
According to Microsoft, there was also a series of invisible performance improvements in the interior of the ASP.NET runtime that should make ASP.NET 4.0 applications work more efficiently and use less resources. These features come without any change requirements in applications and are almost transparent, except what you get through upgrading to ASP.NET 4.0.
There is a PDC presentation (which I cannot find now) that claimed the same thing. I would suggest that YMMV is depending on your implementation, anyway.
Obviously, ASP.Net 4.0 will benefit from any areas of the .Net 4.0 runtime on which it depends, which is improved. It's usually safe to assume that a new release will make things no worse ...