I am having this exact same issue. Since Microsoft is no longer supporting Windows 7, which has been working fine, we upgraded to Windows 10 with Parallels 15. Now runs so slow almost not useful. It is not the hardware: iMac (Retina 5K, 27-inch, 2017); 4.2 GHz Quad-Core Intel Core i7; 64 GB 2400 MHz DDR4 What is the fix?!