Details are available on how to create Virtuoso query compilation reports from the SPARQL endpoint and also how to obtain query explain and profile plans for SPARQL queries from the Virtuoso isql command line tool. With documentation on how to analyse query plans available and should serve as a starting point.
Virtuoso query logging can also be used to obtain a large number of execution statistics as well as the full query text and execution plan with per-operator timing and cardinality information.
Other useful links are: