The preheater is a very important parameter to support the soldering.
You should always distinguish between a single sided assembled or double sided assembled PCB.
For a single sided assembled PCB you can always set the preheater to „follow 0°C“. This makes the soldering very efficient. The oven can reach a maximum heat climb of 80°C per minute. This varies according to the design and construction of the board itself including the amount of copper on the board, the size and number of components on the board, the presence of heavy copper inner layers, the use of thermal reliefs on the vias, even the colour of the solder mask.
Setting a fixed temperature of e.g. 175 ° C is more suitable for double-sided assembly, as the bottom of the board is less heated.
The two pictures below show clearly that the warming of the PCB varies a lot due to the preheater settings.