This was first reported as a comment [0] to ODLPARENT-86 but that is a milestone bug as a possible fix, this is a separate description of current symptom.

Compared to Carbon (Karaf 3), Nitrogen (Karaf 4) Distribution takes much longer to start. Probably the cause is the new way of generating features from pom files, and inefficient algorithm used when parsing feature repositories present in Distribution.

An example is karaf.log [2] showing it took 4 minutes to parse feature repositories even though no odl feature was being installed at that time (only standard and wrap features):
2017-09-14 10:40:37,641 | INFO | d95-9b2f61aab805 | Main | - - | Karaf started in 0s. Bundle stats: 10 active, 10 total
2017-09-14 10:44:32,413 | INFO | pool-1-thread-2 | FeaturesServiceImpl | 6 - org.apache.karaf.features.core - 4.0.9 | Adding features: standard/[4.0.9,4.0.9], wrap/[0,0.0.0]

While this is just a performance regression, this has required a workaround [1] in distribution offline test job, and in the future this might also limit size of features able to finish SingleFeatureTest in time.

As this is probably a bug inherited from Karaf project, version upgrade will be needed to fix this.

[0] https://bugs.opendaylight.org/show_bug.cgi?id=8258#c3
[1] https://git.opendaylight.org/gerrit/62903
[2] https://logs.opendaylight.org/releng/jenkins092/distribution-offline-nitrogen/41/karaf.log.gz