I experience CPU bottle necking with my AMD GPU due to population i.e. other riders. a population distance 5-500 meter slider would be nice if it would help increase FPS for lower spec systems.
I believe Zwift already employs a 100 rider limit, however, a slider like you say with the option to reduce it down further could be useful. I use Zwift on my phone and certainly notice the battery draining quicker when riding in a big bunch due to all the extra graphical computation.