You're entirely correct - VoltDB is very OLTP focused. Its predecessor, H-Store (decent paper here: http://www.vldb.org/pvldb/1/1454211.pdf ) ran on a single-thread-per-processor model that was very performant for short operations, but could obviously cause high latencies in the presence of long running operations. Presumably Volt runs on a similar model.

edit: ah, MichaelGG has explained it much better in another part of the comment thread!