@ryanlecompte - I don't know for sure, but can say that my experience with Vector vs. ListBuffer looks much more like what @markhamstra got.  I've never seen ListBuffer seem so expensive in comparison (though I haven't tried it on very tiny lists).  Maybe you're running near max memory?  Taking 139 seconds is not something Thyme ever normally does on a benchmark taking milliseconds.  It would only do that if there's huge variability in the results--in such a case it just keeps upping the scale of the test until things stabilize.  Do you need the resulting Seq to be indexable?