Speed sensor lag, speed discrepancies in strava

I changed bikes recently and with it switched to the duo trap speed and cadence sensor. I haven’t set up the cadence part since my pedals report cadence. I thought my garmin was set up to connect to the sensor properly but I notice that when I stop at lights it takes a bit to show that I stopped and auto-pause. It is just annoying because it is making me think that the device is still using GPS and not the sensor for the speed data. Is there a way to check the source of the speed data from the files in retrospect? Also any other suggestions to get it to update the speed without lag/ autopause faster?
The second interesting thing as I was trying to figure this out is that on strava the average speed is different from that in analysis. has anyone seen that before? Its a curious difference and would be interested to know why. Is it calculated differently? Below are some screen shots of the discrepancy.

vs under analysis:

I have a Garmin speed sensor paired to my 530 and if I remember correctly it calculates the speed and the distance and gps is only for the map you see after.

Garmin never asks for a wheel diameter so they must calculate that with gps some how. Maybe they use a combination?

It auto calculates usually in first ride after about 1 mile

There can be some slight differences in assumptions between Garmin and Strava as to what constitutes a pause. You may want to look more carefully at stopped v moving time between them, as this will affect the average speed calculation.

That is right. I do distinctly remember receiving a notification that it calibrated my speed sensor against GPS after a bit of riding the first time I connected it. Do you know why It is lagging so much when I stop at an intersection? Is that normal?

I dont think you have a garmin one so I cannot comment. Mine works great and I have the new version but have had the magnet one in the past.