Only just seen this thread… I’m using the Wahoo RPM bluetooth speed sensor as well. I’ve found there’s lag, but not to the same extent as you (I’d say max lag of about 5 secs going from low to high power).
When a high power interval is approaching I tend to start settling in to my required gearing/cadence about 10 secs before it starts.
The lag is the main reason I put in a feature request for the app displaying the exact target power of the next interval… so you’re able to get gearing/cadence set quickly for the interval.