When CP used to operate the passenger service to Cambridge, it was 61 minutes for the morning run, and 65 minutes in the evening run, from Union to Galt using RDCs in the 1969 schedule, stopping only in West Toronto. The current GO service from Union to Kitchener is 120 minutes.
If they could match the 1969 schedule, they'd be 55 to 59 minutes faster. So a 50 minute saving is quite achievable, if you run an express.
The old schedule shows 25 minutes from Milton to Galt. So if 57 minutes to Milton, then that would be 82 minutes to Galt. Still a 38 minute saving if you stop at all stops.