I am still having the same problem of a 3.9MB file taking 90 seconds to open on Sigil v7. I could not find the EPUB version of the userguide to measure how long this takes to open.
I had an warning message of high CPU usage when trying to open a larger file and monitored this when opening the 3.9MB file. The CPU usage shot up to 100% within seconds and remained at this for over 1 minutes. According to the Windows base score and another test tool my PC is healthy. It has a 4GB RAM running Vista 64.
Does anyone have any suggestions?