Fromager bootstrap creates a build environment and installs build system requirements for every package. It takes a few seconds for package with just Hatch or setuptools as build requirement. For packages with build dependency on Torch, the step can take over half a minute. 

For Fromager wheels, this step should not be necessary. Fromager wheels contain additional files with build system and wheel build information.

• delay "BuildEnvironment._createenv()" until the build env is used.
• improve "Bootstrapper.bootstrap", so "Bootstrapper._prepare_build_dependencies" does not need to install build dependencies when it encounters a Fromager wheel.