Sailing PD calculates your boat's speed against the polar with the target speeds. Performance including current or without it, performance against water speed from the log, performance into the wind. Every second.
Target heel comes from measurement certificate. Note that actual heel is more than target with TWA < 100 and smaller than target when TWA is > 100.
Not enough meat on the rail upwind, and not enough sail going downwind....
Performance suffers more downwind.
TWA and Performance after 24 hours of racing. Data from the PDlog.
When the TWA gets to 100, Performance starts to drop (see arrow). Too much wind for spinaker, no asym, the genua suffers behind the main.
SPD on a W10 mini PC, feeding its data to a Raspberry Pi. Signal K on the Pi converts the SPD-data into NMEA2000 and adds labels. The result goes via N2K to a B&G display, showing Performance %, Optimum TWA for the TWS and VMG Performance.