This is mostly a problem with the computational capability of the devices running Zwift,
and possibly inefficient algorithms from the App itself.
If you are in doubt, but want to ride the event anyway try to avoid the messy starts
this will ease the demands on your device.
Zwift is trying to render for you only the closest 100 or so riders.
When gaps are established and groups are formed you device has to care only about those 100 riders.
So it doesn’t really matter how many there are overall.
But at the start you might have almost 10000 at one spot and this is a big ask.
What works for me:
1. Get a front row position.
Race positioning opens 30 min before the start.
Log in as close as possible to that time and just leave you avatar there.
This will save you a spot close to the front.
2. Start pedaling hard 5-10sec before the start.
I would go for 130% of FTP, but best not below 300 watts for all categories Events.
2. Try to keep with the front for the first minute or two.
Zwift starts are basically sprint starts, so gaps form in the first minutes of the race.
If you keep for a while with the front due to the power of drafting they will put
big gap between you and the rest.
The last big event I rode was the Haute Route Watopia.
In my group we had 9853 riders stage 1 and 4638 rider stage 2.
The first stage I had issues, the second I did this and it went fine.
I am using Samsung S9 Android phone, something that is far from a dedicated gaming PC.