There aren't any hard and fast rules for speeding it up, and the performance issues that you're seeing might be specific to your app.
If your bootstrapping is taking ~75 seconds, that sounds pretty long. I'd take a close look at whatever you have in your Bootstrap.groovy file to see if that can be slimmed down.
Do you have any extra plugins that you might not need (or that could have a major performance penalty)?
This might not be a possibility for you right now, but the speed improvements in grails 1.1.1/groovy 1.6.3 over grails 1.0.5/groovy 1.5.7 are fairly significant.
Another thing that really helps me when testing, is to specify only integration tests or only unit tests if I'm workiing on one or the other:
grails test-app -unit
grails test-app -integration
You can also specify a particular test class (without the "Tests" prefix), to run a single test which can really help with TDD (ex for "MyServiceTests" integration):
grails test-app -integration MyService
In grails 1.1.1, bootstrapping with 5 plugins and ~40 domain classes takes me less than 20 seconds.