As a rider, I haven't noticed the Lyft app to be slow or especially bloated, so it will be interesting to see if it is significantly faster once the update pushes through with the driver portion removed.
I doubt it. If you aren't a driver I think it would be senseless to be running driver code at the same time as rider code. Maybe a little less disk (flash?), but I'm not certain. I'm neither a mobile dev or a Lyft employee.
