Two related problems related to the complex quadratic polynomial $f_c(z) = z^2 + c$ and Mandlebrot and/or Julia sets:
find an external angle $\theta_c$ for a complex point $c$
find a complex point $c_\theta$ for an external angle $\theta$
Currently I can do both of these by tracing external rays (outwards for 1, inwards for 2), but it is asymptotically too slow to be practical: $O(n^2)$ where $n$ is the sum of the preperiod and period of the external angle.
Are there better algorithms? (What is the asymptotic cost of the Spider Algorithm, for example?)