In digital design at least you have delay of one inverter that impose a maximum theoretical limit on switching frequency and speed.
In analog, you have the same limit but you can determine it with the fastest possible amplifier. Our master says that a folded cascode amplifier with good layout is the fastest amplifier that can be implemented.
At .18 a invertor cell which have one input and four output which dely is 20~50 psec, so if the freq is 1G
then from DFF to the next Dff is 1ns, so at the DFF to the next DFF it just can have 20~50 cell. If you can write rtl good , it will work good. But i feel the freq 300~600M is the freq of limit .18.
Max Frequency that can be acheived is a function of the way you implement your hardware.
So you should look into the longest combinational path that you have in your design. This will control the max frequency as compared to the process(0.18u)