For projects which make heavy use of <scope>import</scope> including in parent POMs, calling MavenProject.getParent (thus DefaultProjectBuilder.build(Artifact, ...)) can be intolerably slow - taking many minutes - even when loading the project (or its parent) via DefaultProjectBuilder.build(List<File>, ...) takes less than a second.
The discrepancy seems to be due to the fact that ReactorModelCache is crucial for the performance of DefaultModelBuilder.importDependencyManagement, yet only one DefaultProjectBuilder.build overload defines a ModelCache. For resolution of parents from a single POM, no model cache is likely to be needed under normal circumstances, but if you are missing a cache when import scope dependencies are processed for <dependencyManagement>, the builder takes exponential time to find these imports.
I do not have a minimal test case for this yet as the known test case involves a large and complex proprietary source base. Patch (baseline is 3.0.4) successfully tested against these sources and shown to reduce getParent times by orders of magnitude.
https://issues.jenkins-ci.org/browse/JENKINS-11362 is the downstream issue describing symptoms.