The matter of heat up time for any sauna will be a matter of personal bather preference and outdoor weather conditions. What I mean by this, is that each sauna bather enjoys their experience uniquely. Some folks want to be in the room from the start up so that they can experience the heat gradually rise from room temperature up to the desired high temperature. Others want to enter the sauna at the exact high temp that they prefer. And finally, everyone enjoys their sauna experience at a different temperature point.
This means that the optimal temperature for each bather will be reached at a different time interval. Combine this with differing weather conditions and you can have a wide range of startup time to reach the desired temperature. It is unique and specific to the bather and the location. Experimentation will be key in getting it perfect for each person.
Ultimately, the same sized sauna heater from one manufacturer should take a similar amount of time to heat up as with any other manufacture. I suppose there may be some slight nuance for each brand, but I would suspect that it would be imperceptible for the average sauna bather. Most importantly, I think, is the addition of the Wi-Fi app to control the heater remotely and allow an easy way to pre-heat the sauna room, no matter the amount of time needed.
As always, get out there and enjoy your sauna room. The more convenient that it is to use results in the bather enjoying the sauna more frequently and enjoying the physical and mental benefits of sauna bathing.