Processing time is the part of a service when the product, not the practitioner, is doing the work. Colour develops, lightener lifts, a perm or lash lift solution sets, a tint takes. The client is still there and the service is not finished, but the practitioner's hands may be free. How long it takes is set by the product's instructions and, for lightener, by what the stylist sees as it lifts.
Processing time matters for two reasons. It is part of the true length of a service, which is what the menu and the diary need to show. And it creates gaps that can be used well or badly. A stylist can do a quick trim on another client while colour processes, as long as both services fit and the colour is checked when it should be.
In practice
A full head of foils takes 50 minutes to apply, 35 minutes to process, 15 minutes at the basin with a toner, and 30 minutes to cut and blow dry: 2 hours and 10 minutes in the chair. The stylist uses 25 of the 35 processing minutes for a $45 fringe and neck trim on a regular, which leaves 10 minutes to check the foils and get the basin ready. In a week with 8 full head colours, that is room for up to 8 extra short services without lengthening the day.
The risk is overlap. A second service that runs long means colour left on too long or a client waiting at the basin. Book overlaps only with short, predictable services, and never treat processing time as buffer time, which is meant to stay free.