I've just upgraded to 2017, and my design is taking 1-1/2 hours to load. Used to take about a minute. Works fine once loaded.
I've identified the problem as being a couple of circuit board models imported from Altium Designer as solid models. All the models are already .sldasm files, yet each component of the PCB takes up to several minutes to load. The whole assembly takes ages.
Even if I manually load a file, and save out to 2017 format, it still takes ages to load.