HYBRID is usually very fast so long as the clusters of ionizable groups don't contain more than 10 or 12 groups each. The code forms the clusters automatically based upon the interaction energies provided in the input files. Groups that interact strongly are grouped into a cluster, while interactions between clusters are weak.
This zip file provides the fortran program, a description of its usage, and sample input, data and output files. The sample files are for 14 histidines with strong electrostatic interactions, so the titration curves are "abnormal".
The code is freely available for academic or personal use. I do ask that you cite the reference above in publications that use the code or another implementation of the same method. If you wish to use it for a commercial application, please contact me.