The water is heated instantly as it passes through the shower by a high output element. Generally speaking, the higher the power rating of the shower, the higher flow rate you will receive at a higher temperature of water (this obviously depends on your mains pressure to some extent). Also be aware that your new shower will require an appropriately sized electric cable to feed it (usually 6mm or 10mm).
Flow rates can very as the incoming water temperature fluctuates through the seasons. With an 8.5KW shower you can expect a flow rate of around 3.5 litres per minute in the winter and 5.1 litres per minute in the winter.