As of llibrary-1.7.14-1.12.2 + iceandfire-1.5.2, profiler reports systematic CPU usage in calls to EntityDataHandler.getEntityData.

Here it is 0.16% for each mob type, so it scales up pretty fast.
Compared to other mods, similar EntityUpdate events are taking less than 0.05%.

While we could look into optimizing the lookup, this looks like an Ice and Fire issue.

What would be the best approach to address that issue in Ice and Fire then?

Alright looking at it again there are some possibly significant optimizations that can be made on our side, however doing a capability lookup every tick on every entity is still significantly slow.
23b9436

Improved in 1.7.15.