Once an Open Access dashboard is rendered, the image is cached for 60 seconds. This means that the server does not do extra work for users viewing the same dashboard, as it just returns the cached image.
There is a new render process for each different Open Access dashboard open. So there should only be one render process if 100 users are connecting to the same Open Access dashboard.
Once a dashboard is closed and no one is viewing it, the process exits after 60 seconds of no use.
I don’t know if this applies but I found one customers install where SquaredUp was installed on a SCOM MS server. We moved to a dedicated Squared server and the render speed increased significantly. The problem seemed to be the IIS worker process fighting SCOM for CPU.
I have found that once we groomed our DW, dashboard load time was improved. I found a really great blog post on DW grooming which you may find interesting, especially if you are looking for performance enhancements, Understanding and modifying Data Warehouse retention and grooming.
I actually suspect the reason I was getting more processes is because the dashboard was not finishing load before another user view. Had 16 of the render process listed in Get-Process on the server.