SOD can use multiple threads in the waveform arm to retrieve and process data. This allows for SOD to stay busy on processing one event-channel group while retrieving the data for another. Setting this to 2 can speed things up. The value defaults to 1. For IRIS DMC usage, see http://ds.iris.edu/ds/nodes/dmc/services/usage/ for information on resource limits. In particular for versions of SOD prior to 3.2.5 you should not have more than 3 waveform worker threads or the rate limiter at the DMC may block your requests, resulting in very slow performance. For later versions of SOD requests are batched so more worker threads can be used. However, diminishing returns sets in very quickly, and many threads may end up being slower than a few.