They basically render only 100 riders around you. This is visible in some weird ways - in the start pen, you see a bunch of riders around you, but a big empty area between that bunch and the start line… that’s where the riders you don’t see are. Once riding up the Alpe, you can see the algorithm at work, with riders in the straights above and below you visible on the map, but not those ahead or behind you on the same straight as you if they are further away. As the pack thins, the radius of visible riders on the map increases… So clearly they’re doing “100 riders closest to you”, where closest is straight line distance and not distance on the road. It’s a simple way to limit both network traffic and rendering workload.
Not sure if it was related, but the list of riders on the right hand side, showing those ahead and behind you, was not working this morning. It already gets reduced to make room for the Alpe map, but it showed only me, no data.