Depending on your line frequency, IoTaWatt samples one input every 24 or 30 ms. That’s the basic hardware capability. The firmware is designed to be an energy monitor, to track energy (kWh). Power (Watts) Is a byproduct. ioTaWatt isn’t a power meter.
IMO if you have a need for subsecond data, you would need to write a completely different firmware that doesn’t have a web server and data communications that can compromise subsecond sampling. Just a straight datalogger that samples and records to the SDcard. There are probably better tools for that already available.