Easy. Its called Instancing and MMO’s have been doing it for the past 20 years or so.
TT event gets 4000 signups (highly unlikely but lets roll with it)
Zwift splits that event up into batches of 60 riders per ‘instance’ of the event course.
thats a 1 hour window for start times with a 20-30 minutes race time for a 10 and 45 - 70 minute (ish) event time for a 25. so the entire event for a 25mile TT would take no more than maybe 2.5 hours from the time the first rider rolls off the ramp to the time the last rider crosses the finish line.
You can bring that event time down further by having an alogrithm analyse each participants zwift history and set the slowest riders off first and the fastest riders off last.
Even though the event is instanced into groups of 60 riders, the results are collated into a single results sheets as a single event.
Not difficult and this is literally ancient gaming server technology.
I feel like a broken record when I keep repeating this same line but Zwift need to realise they are making an MMO with bicycles and runners, not a ‘fitness’ application.