Unless you have a big server you will likely hit some resource limit trying to update 500 MLS sources at once via cron.
Instead, you should look into a job server that maintains a queue and then define n
number of workers to grab the top item from the queue and begin processing. Once a MLS is completed that worker is free to do another MLS. Based on resources/servers available you can tweak how many parallel updates can run via workers
Look into something like http://gearman.org/ or http://resquework.org/
Practically every alternative can be found here: http://queues.io/