It needs appropriate calculation. Fuel costs per hour may be saved with reduced speed, but per mile will follow a curve, just like they do if you gun it at maximum speed. And you are going to need more hours for the trip.
Very many of aviation's costs, such as crew, interval-based maintenance, even some overheads such as leasing and insurance, are based at least in part on hours. So to provide the same service can increase these costs. The only approach is to have an accountant work it out.
Losing out on achievable sectors per day is a lesser issue, as many turboprop operations, certainly on a business day, are one worthwhile return trip morning, and one evening, and the rest either marginal stuff or standing. It would be poor if the morning round trip previously 0700-0745/0805-0850 became 0650-0750/0810-0910, as the user perception of a before 0700 departure on the first leg, or an after 0900 arrival on the second, may lose you some custom. Don't think that cheeseparing of costs allows you to maintain the same revenue.