To calculate the time between two samples of an asynchronous signal you need to first make a formula, which will give you the time when a sample comes on the asynchronous channel. In the formula you must set the time axis to Async and for the channel select the async channel which time between two samples you want to calculate.
The next formula will calculate the time between two samples. In the formula you must subtract the current Async time with the Async time from the previous sample.
You can now make a new formula where you divide 1 with the "TimeBetweenSamples" channel to get the sample frequency and use that formula in basic statistic to calculate the average asynchronous sample rate over a wanted time frame.