As of ProjectOzone3 3.0.38 / NuclearCraft 2.13e-1.12.2, lag is observed in
RadiationCapabilityHandler_attachStackRadiationCapability
RadiationCapabilityHandler_attachArmorDefaultRadiationResistanceCapability

As seen on 5 mn profiling with a single player using 2 hoppers to transfer cobblestone:

It appears an hashmap is used on objects with no defined hash() method, defaulting to the slow default one.

Full profiler report is https://sparkprofiler.github.io/#cc4afQiY0S .

Have improved stack capability attachment to query these stats only when asked for :)