I can believe the 45-60 mins Sydney CBD to WSI in non peak times, but in peak hour this could blow out to 90mins easily. The M5 can be diabolical at times and you still have to get through inner city rds/streets to get onto the M5.
The SYD to WSI is also an interesting one, it has to go via Central Station/CBD first (SYD-CBD-WSI or WSI-CBD-SYD), its not a direct service. The more direct route which involves no back tracking would be WSI-SYD-CBD/ CBD-SYD-WSI, but understand this would blow out travel times to/from the CBD.
When the new Western Sydney metro opens (early 2030's), you might be better off getting the metro to Parramatta (20mins from the CBD) and then getting a bus. It will be quicker than the train from the CBD to St Marys and then getting on the St Marys to WSI metro.
For anyone living on the North Shore/CBD/Eastern Suburbs/Inner West, getting to WSI takes some logistics and time if not self driving/taxi/ridesharing.