The baud rate of the output would cause the issue. If you are trying to send out a lot of effect changes over a lot of channels, the limitation would be the refresh rate at which you could send out the updates. Not a software problem, but a physical databus problem. You just have to be careful if you start putting a bunch of 25ms time intervals in with a lot of effect changes. 50ms can fit 512 channels without a problem. 300 channels with 25ms time intervals with a lot of effects across the channels should work as the maximum transmission rate of all 512 channels is around 23ms. Slower baud rates would take longer.
If he is using open DMX adapter then things change since that adapter doesn't have the ability to "store" and send the DMX data like the Lynx or DMX Pro.
EDIT: Also, please note that the LSP DMX output plugins are setup at a refresh rate of 30ms. You can change this value to lower, but we felt that anything faster than what the normal human eye can see (32Hz) is probably not needed.