I live in Michigan, and it's not uncommon to go 55-60 on roads that are not highways. It's extremely annoying to go 50 mph and see all normal roads disappear. Annoying when you want to see upcoming intersections while driving on a 55mph road. The other day I was driving on a 'normal' road in a somewhat-rural area of Michigan and all roads on the map, including the one I was driving on, disappeared. It was all gray. Waze was effectively useless.
I hope the developers plan on adding preferences to disable the road-hiding feature or set these zoom threshold levels manually in the settings. I use an iPhone, so I can edit preference files by hand.
Perhaps a compromise if system performance is concerned is to do as suggested earlier, allow roads to be hidden, but always show the road you are on and the ones immediately interesting in front of you, regardless of their 'rank'.
I get the impression that the developers live in large cities and don't expect people to ever drive faster than 45-50 unless if they're on the highway.
Perhaps I'm repeating what's been mostly said, but this should really be more of a priority, in my opinion. It's enough of a problem to make me want to open other apps to have more context of where I am sometimes.