Hi Brenden,
There is a CVar that is used to force a full build when there is too much work. You might want to grow that number to keep time slicing going on. Check for AccelerationStructureTimeSlicingMaxQueueSizeBeforeForce in the Chaos code. The default is 1000 which is likely easily hit in your case. This will defer the creation of the internal structures so might cause other problems.
Martin