I tried Iris via Tor exit nodes. It is very slow and unreliable. I switched to VPN and Iris is a lot better now.
I wonder if I could use Tor in the desktop application https://tpo.pages.torproject.net/core/doc/rust/arti_client/index.html
Discussion
I have used Iris + Tor Browser over VPN from day 1. It was indeed laggy at times (maybe because I have 40+ relays), however recently it became much better. Need to try the same set up with a small number of aggregator relays.