Magento is a complex product, and this complexity comes with a downside: On a non-optimized hosting environment Magento can be quite slow. This whitepaper discusses options on how to optimize things so that the Vm2Mage migration goes as fast as possible.
Make sure the Magento cache is fully enabled. Refreshing the cache should be done after the migration has been completed, but having the Magento cache enabled optimizes the Magento application.
Configure the Magento indices to have the mode set to Manual Update instead of Update on Save. During the migration, this will give the message of indices needed to be refreshed. Ignore these messages, and simply refresh all indices once the Vm2Mage has been completed. Make sure to set the indices back to Update on Save once you're finished.
Disable the Flat Catalog for Products and the Flat Catalog for Categories.
Within the Joomla! parameters of the Vm2Mage component, you can set the batch-size. Having a batch-size of 100 items makes sure the Magento application is started up only once. Make sure to set the PHP memory_limit high enough though. Having a temporary memory_limit of 1Gb should not be strange.
Make sure Magento runs smoothly in the hosting environment. The migration is mostly database-intensive and MySQL tuning is mostly done by dedicated memory to certain parameters. Refer to Magento whitepapers to see Magento's recommendations on MySQL tuning. Note that the query_cache is useful for optimizing queries that occur frequently. With a Vm2Mage migration that's less useful, because most of the queries you run only occur once. Instead, pay attention to tuning the InnoDb paging and buffering.