From a compensation economics standpoint, property management contracts are such that they don't align the interests of the owner and the property manager.
Property managers are contracted to rent out the property and manage it to get , say, 10% of the rent. If they don't rent it out, yours is one of the 100 properties they manage, so it doesn't move the needle that much for them in terms of overall profitability. If they do rent it out, they get straight 10%. The money they make is from the 10-11 months of the year they rent out the place.
Regarding performance measurement, there are not quantifiable measures to evaluate them. Normally, one should benchmark a manager's performance to the other managers in the area and give them commission and if they do better. i.e. if it takes 1 week to find a renter in an area X for $800/month and if your manager does the same, then there is no benefit to having a manager as you could do that too. In an ideal works one should take tenant surveys and see if your tenants are relatively happier than other tenants in the area.
Plus there are low barriers to entry in this business. It requires very little skill and most of it is not specialized skill.
If we as landlords could write quantifiable performance, prop managers will quickly become useful.
My 2 cents.