It’s because Tor network had long time not enough servers to run a lot of traffic. 2011 Tor had a capacity of 12.8Gbit/s, while 8Gbit/s where used in average (that means bad internet on prime time). This changed dramatically in last few years. 2019 it had a capacity of 184,64Gbit/s … 2.5 years later it had a capacity of 775Gbit/s and a usage of 275Gbit/s. I don’t think that updating PureOS via Tor is a problem for Tor. It just takes some time longer. My source is the German Wikipedia (the English one had not the information).
I’m just not sure if that makes so much sense for privacy at all. But that’s not to me to rate that.