PowerGraph open source code is soon going to be deprecated as we are switching to GraphLab Create, which is a our newer system.
HDFS most likely will be slower than NFS. This is not an issue of GraphLab but an issue of your storage layer. There are numerous parameters which may affect HDFS speed. Including: replication factor, compression, block sizes etc.
GraphLab Create can scale in a single computing node to input data files which are terabytes in size. Working with a local disk will give you the best performance for this scenario, however we do support HDFS in GraphLab Create. You are welcome to try it out.