I was about to post a topic on this exact same thing.
For stage 3 there were 4,900 participants at the 7pm GMT session. I cycled for 30 minutes and the ET timer was at approximately 9 minutes and 30 seconds, it recorded only 5.4km in that time. I was getting around 1 frame per second in the game, and even exiting the ride took about 5 solid minutes as the interface was completely unresponsive. Meanwhile, my laptop was generally fine, and responsive, just the zwift application which seemed to have an issue.
I actually missed the first stage, so I already have 1 catch up day to commit to, and now this issue on the third stage.
I’m contacting support about this, it’s very disappointing. This is my second week of Zwift, and my first week as a subscriber, the TDZ was a big motivator for me to subscribe but it’s absolutely unusable for me.