It doesn't say it is just 6m rows. It is 6m patient, which only hints that one of the dimension tables is 6m. Facts gathered in patients might be significantly larger. Also, my experience is saying, if you have hundreds of queries running simultaneously, it is not the volume of data that can be a bottleneck. Depending on the system it can be anything, starting from acquiring lock on a record or initiating a transaction.

I work for a large healthcare org and from personal experience these things can get large. 6m patients is prob 20m encounters, each encounter may have 10 different kinds of meta data (each one it’s own table) and each kind has 10-100 rows. So really you are doing joins between 10 tables each with ~1-10 billion rows. It actually does slow down even running on expensive hardware.