How to diagnose eye diagram slope on physical impedance problems?
well, you say the cause is an RC thing. Answer yourself this question: is it the R or the C that limits the slope? It's both. so, your signal source can't charge the C infinitely fast due to the R, and the C is what needs to be charged before the output reaches the input voltage.
Thus, your impedance has both a real and an imaginary part – it's an RC, alright.