I tried uninstalling Sigil 7.0 and re-installed Sigil 6.2 64 bit.
The epub file taking 90 seconds to open with version 7 now opens in 10 seconds. It looks to be the CPU as the task manager shows on the processes window that version 7 is using 100% of the cpu within seconds and then seems to open very slowly with the CPU at 100%. Version 6.2 only reached 98% for about 1 second.
I think I will stick with version 6.2 for now.