How do I get an acceptable performance and latency on the MS Windows platform?
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Make sure your Windows system is set up for Audio. Make sure the overall system performance is OK, install updated ASIO drivers for your soundcard or the "ASIO4All Drivers":http://www.asio4all.com/ when no ASIO is available from the manufacturer. There is also an "ASIO driver for WDM kernel-streaming compliant soundcards":http://www.asio2ks.de/
Read Native Instrument's "Windows 7 Tuning Tips for Audio Processing":http://www.native-instruments.com/knowledge/questions/847/Windows+7+Tuning+Tips+for+Audio+Processing
Also read RME's "Tuning Tips for Low Latency Operation":http://www.rme-audio.de/english/techinfo/lola.htm (slightly older, possibly outdated)
USB and persitency
If you have the problem that you can only plug in your external sound card to one specific USB port you might want to make the COM port assignment of the system permanent to this device. Here is How: goto control panel -> device manager -> com ports -> properties -> port settings -> advanced -> comport number there you can permanently set the comport number for that device.