According to that live-tweet feed, the train departed Kitchener precisely 30 minutes late and followed the schedule of the subsequent express train (arriving Union 9:43). GO pulled another train out of the yard early to run the Kitchener-Toronto trip which arrives at 9:13, which the London train should have filled.
The GO train then arrived in Union precisely 30 minutes late from the perspective of London-Stratford passengers (on time for Kitchener-Toronto passengers), clocking in a brutal 4h23 travel time from London to Toronto (45 km/h average).
But a 30 minute delay is still better than VIA 84, which seems to be on track to arrive 49 minutes late today:
The train is currently sitting at Wice (Pearson Junction) presumably waiting for an UP Express train to cross. This is what happens when you enter a busy tightly-scheduled rail corridor at unplanned times.
(Times are 6h off because my computer is set to Central European Time)
Yet again, the VIA train arrived in London on time, but accumulated a major delay before St Marys, which then continued to get even worse until Kitchener. This supports our theory that VIA's 1h56 scheduled London-Kitchener travel time is way out of date, and GO's 2h12 estimate is far more representative of the current state of disrepair.
Delays along the line:
0 Strathroy
+2 London
+23 St Marys
+31 Stratford
+39 Kitchener
+38 Guelph
+39 Georgetown
+39 Brampton
+42 Malton