It was, although it got bitten at one update explicitly by the GC: entity positions moved from a bunch of arrays, to a single Position() class for each entity,

It was, although it got bitten at one update explicitly by the GC: entity positions moved from a bunch of arrays, to a single Position() class for each entity, leading to memory allocations that murdered performance. But then again, that's a problem you'd get without GC too, just maybe a bit less harsh/unpredictable.