Hello,
Thank you for the detailed explanation.
Please, perform the following few tests also
1. Switch to default theme and compare time loading with our Basel.
2. Disable all additional plugins except ones that come with our theme.
3. Does it happen only on the home page? All other pages works fast?
3.1 If yes, then try to remove all Visual Composer elements from the page one by one and measure loading speed.
Write us about all the results.