The rate at which the triples can be dumped would be dependent on the hardware being run on, i.e., processor speed, memory, etc.
The state of the Virtuoso database would also be relevant in terms of memory allocated to Virtuoso for hosting the triples and dumping them. When the dump is being performed you should run the status(); command and in particular check the number of Buffers allocated and used, ensuring all the buffers are not being used which would result in swapping between memory and disk which will always significantly degrade performance:
SQL> status(); REPORT VARCHAR _______________________________________________________________________________
OpenLink Virtuoso VDB Server Version 08.03.3314-pthreads for Mac OS X as of May 24 2019 Registered to OpenLink Software (INTERNAL USE ONLY) (Personal Edition, 500 connections) Started on: 2019-07-16 13:15 GMT+1 CPU%: 0.05
File size 400556032, 48896 pages, 14588 free. 20000 buffers, 3523 used,
. . . Setting log_enable(2) to turn off transaction logging and turn on autocommit would also reduce the memory requirement and speed the dump process.