Found some optimizations in the general evaluation framework related to blank cells in rows beyond the last defined row of a sheet.
I don't see any issue with passing a bit of context down deeper into this framework, as it's all POI-internal and only had one calling path.
See the above bug for the performance analysis. Not specifically related to VLOOKUP, but improves that case by more than 2/3 as well.
git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1817252 13f79535-47bb-0310-9956-ffa450edef68
