So there were some changes to the Fusion 360 presonal/free version which introduced this problem. Most people are solving this issue by limiting the Z feedrate in firmware. You can execute the following g-code once. The change is persistent:
M203 Z4 ; Limit the feedrate of Z to 4mm/s (240mm/min) M500 ; Save the changes to the EEPROM
This places the limit on only the Z asix, so you can up the feedrate for the X and Y. This is somewhat of a brute force solution. The better fix might be changes to the guffy post processor. Take a look at post 162 by Don in this topic.