Well in our case we use 15 minute slot intervals but people will usually book for an hour or more in one reservation. Automatically adding the next available 15 minute slot after the reservation time would suffice for our needs.
e.g. A reservation made from 10:00-12:00 (8x 15minute slots) would actually become 10:00-12:15 (9x 15minute slots) automatically.
While the user only needs the item until 12:00, the earliest available time that someone else could book the same item would be 12:15, which means we have 15 minutes to prepare the item for the next person. An option to automatically add X number of timeslots to a reservation would be nice. This way if we needed 30 minutes to prepare the item we could allocate the following 2 slots after the reservation as preparation time. In an ideal world it would be nice to identify which parts of the reservation have been automatically added as preparation time too. That way people in charge of reservations won't just assume that the reservation has been made until 12:15 rather than 12:00 (using the example above).
I do see what you mean with the other option of having a custom time interval shorter than the duration of one time slot but I expect that is probably harder to achieve? That method is not necessary in our case anyway but I can't speak for others who may use larger timeslot intervals.
If any of this doesn't make sense just let me know and I'll try to elaborate as I feel like I'm starting to waffle on a bit here