Possible causes of frame drops include:
- Incompatible USB controllers. Intel and NEC USB 3.0 chips usually work best.
- Low quality extension cables. Check whether frame drops appear when Kinects are connected directly to a PC.
- CPU overloaded due to too much data. Check CPU load.
In any case try depth-only video mode - this will reduce data rate. Color is not needed for tracking.
Color is required for calibration though. However, calibration is more tolerant about frame drops. So you should be able to do successful calibration even while having frame drops.