Here are some test results from the test file you sent us. Hidden line rendering is faster in 11.5.1 than in 11.0.1. Can you give us some steps on how you are getting different results with HLR ?
Results w/OSX(10.4.3) on a Dual 1.8Ghz G5/ 1GB RAM:
HLR test: 11.0.1/ 3D conversion res 'Very High' --> 30 secs
HLR test: 11.5.0/ 3D conversion res 'Very High' --> 20 secs
HLR test: 11.5.1/ 3D conversion res 'Very High' --> 20 secs
the other issue with the Hidden line rendering being slower when the 3D Conversion Resolution is changed from 'Low' to 'Very High' is something we are currently investigating.