Ed Glas and Steve Carroll showed us the performance lifecycle and it looked a lot like the development lifecycle. They wanted to emphasize that in every step of the development cycle performance is a to be taken into account.

During a short demo with a One Last WalkThrough Before Launch Test scenario they showed the new diagnostics and profiling capabilities of VS10. Some of it was alreade shown by Cameron Skinner (see this post).

When gathering requirements for your application also gather requirements for performance.

In the design phase a prototype can help verifying that the performance specifications are met.

It helps to focus on key scenarios to make those perform best.
Run tests eagerly and measure the results against the requirements.
Isolate components (and save testresults as proof) so that individual components can be blamed or not.
Pre-production testing is very usefull. A development environment always perform because of the limited size.

VS10 has JavaScript Testing and Profiling now!
Reports can be generated in Excel with graphics support.
A comparison tool is made to compare a baseline performance profile with a regression performance profile. Very powerfull!

Avoid reflection. Reflection is powerfull but slow.
Garbage Collection is a source of performance problems. Bad Allocation Paterns are Midlfe crises, managed leaks and large object heap. Large object heap is mostly used in corporation with the use of ViewState.
New problems are .NET locking problems caused by multi processor machines and multi-core processors.