Just the messenger. I won’t pretend I understood the full discussion because UE4 Slakcers is full of plenty of smarter people than me - but that was the consensus. It’s not about the size it’s about the indirection cost. A reference is still a lookup to some other location in memory, which could potentially mean a lot of hopping around. I’m sure the difference is practically negligible.
Most routinely-used engine functions use const value (const FName Value) - such as for bone names in animation, body names for physics, attach locations, data table lookups etc. This is all stuff that is used in high volumes at runtime. If in doubt, follow what the engine does. At the end of the day, I seriously doubt any of this really makes a difference to the average game.