Yes, I only play standard, size, speed, continents.
This decay is the same on all difficulties
First impressions on a high level are a pain which is why early empire and its open borders is very useful. Also an early joint war, do not stand there and let people joint war you, joint war someone else for no negative affect and a +5 with that civ for the joint war that looks like it decays a point every 40 turns... At least the first point decays that slowly... This also may explain why you get joint warred a lot, its the civs making better friends with each other.
Also it looks like a civ will not joint war you if neutral, it may not even declare if neutral, I dunno, makes sense. So trade that cotton that has no use for 40 odd turns anyway.... That trade sadly decays very fast and seems random in amount but hell, anything to counteract those bad first impressions.
I might add this is what is important on emp-dei early game. It is designed so a skilled player can maniplulate what happens to a degree rather than saying "everyone hates me so I am going to kill them all".... Or "diplomacy is broken"....things I see way too often in the forums. Firaxis has been manipulating diplomacy in all patches to try and get us to play it properly, I'm trying to learn it because it does seem to really help and also is a different perpective to play that keeps things interesting and complex