Or in other words, what is it currently that is making this computationally heavy, I expect the physics code or is it the visualization part ?
* you have the per-segment property I/O (at least two, possible four operations per segment) - if you want a detailed 50 m rope with 5 cm segments, it's 4000 properties per frame, go figure
* in addition per segment (dependent on the strategy you use to compute) the rough equivalent of a PID controller to solve the wave equation
Dependent on the level of detail you want for the second, it can be less or more than the first, but I am fairly sure if you go realistic and solve 2000 JSBSim PID controllers, that's going to hit you badly.
so that means, that for say 30 segments, each 3D position is computed separately and then placed in the scene accordingly ?
Like you say, I also have no idea how to account for turbulence/collision detection, or hitting a water surface...
Yes, except 30 segments don't get you far with a rope (even 90 look pretty coarse). And, again, dependent on the level of realism, you'd need one terrain query per segment.