SnowMirror itself is very fast and has been fine-tuned for performance. However the overall performance hugely depends on performance of the ServiceNow instance which varies a lot between ServcieNow customers. Using one of the fastest ServiceNow instances in the world SnowMirror was able to download a 500K+ table in about 5 minutes. However it is just an initial table load. All other runs are incremental load. Even on the biggest instances the update takes small number of minutes. Typical SnowMirror user synchronizes data once a day at night so the performance is not an issue.
By default SnowMirror downloads three tables in parallel. The number was carefully chosen according to the average instance performance. If you instance is faster you may want to increase this number (8 tables in parallel might be the maximum value making sense). Use the following properties
snowMirror.quartz.threadCount and serviceNow.httpMessageSender.max.connections.
