The call to ClassLoaderDataGraph::clear_claimed_marks() during initial mark in g1CollectedHeap.cpp:4420 is misplaced.

The call is performed after taking the start time for the evacuation phase, but before starting it. So the serial execution of this call is counted towards the parallel evacuation phase, which is not right at all.
You can also not get a direct reading of this time, one needs to do some extra calculation for this.

Move the call outside of this place, best into the parallel phase where the work can be hidden by parallel threads, and add some timing for it.