That's the patched saving method. As far as I can tell, those aren't lag spikes - it's the game saving.

That code is only called when the game makes a save, either user-triggered, on the standard fixed autosave interval, or when this mod detects a negative letter arrival. Is your fixed autosave interval set to something super low maybe?

Only other thing I can think of is that I spot a potential conflict with Real Ruins in your log - they both try to patch the save method and could interfere with each other.