I am not to worried about a lower graph rate. The proportion of block subsidy to the value of transaction in grin stays the same. Meaning an attack is not anymore economically attractive now than it would be with a 10x higher difficulty and hash/graph rate. The whole principle of PoW is that intensives protect the chain since any attack would mean a certain waste of money and resources.
This only leaves the option for a 51% FUD attack, meaning there is the pure objective of causing fear in the market. 0.51*6000 = 3000 Graphs per second. Lets say this attacker would have a super low price for G1 chips of 100$, for a graph rate of 1.2, that would be 100$*3000/1.2 = 250,000$ for the hardware alone at least and then you still have the electricity cost. So a FUD attack would be quite costly.
Alternatively someone could rent GPU power for an attack, but we have grindefender.online 
[REF] which should start mining the other chain when an attack is registered making an attack both obvious and costly. I see the website is not online anymore @bladedoyle @Paouky. Is grin defender still online?