Your run-times seem significantly faster than mine are - mine are closer to 7500 seconds for 0.05 and 128 threads. As for run time measurements, are you using clock() to measure the time? I found when using that it seemed to actually accumulate total...

Likewise - 10000 took around a second, and for me doubling the size led to a 10x time increase, so mine will take somewhere around 3 hours Feels quite long however, it will make repeated tests and comparisons difficult