I haven't yet seen this pop up on Android Auto, but good to know it's there. I used to see it fairly frequently when I just used Google Maps on my phone. Haven't done enough navigating in the new car to know if this comes up.
As a workaround, when navigating via Google Maps in Android Auto, you can bring up the options and show alternate routes, and it'll show up to three routes (including the current one) and whether they're faster or slower. It seems to be updated pretty consistently, as I could see the "x minutes faster/slower" info change if I left that view open.
I was periodically checking this on Monday during my last long drive and even though a different route was sometimes faster, it didn't give me any pop-up alerts. Maybe Maps has some threshold where the alternate route has to be consistently faster for a certain number of minutes or something before it alerts you.