Improving CI build time (.NET)

We are developing an application framework + "plugins" using TeamCity as a CI server.

Project Details

1. 4 Visual Studio solutions
2. ~70 projects (and increasing)
3. Currently running 2 builds using TeamCity: CI and FULL build.

CI - triggered on every commit.

FULL - runs nightly.

I would like to improve the performance of both builds (especially the CI build, as it needs to give its output as quickly as possible).

Are there any guidelines in general on what can be effectively and easily improved?

The build process simply builds a .sln file and runs some unit tests.

Directions Considered:

• MSBuild parallelization
• Overriding CopyFilesToLocal

Not sure these are applicable/will result in a performance gain.

I am looking for more ways to improve the build time (which takes around 3-4 minutes).