Once started, from the windows start menu, IntelliJ takes a very long time to start?

You can disable the antivirus or exclude IDE installation and plugins folders from the antivirus monitor (https://intellij-support.jetbrains.com/hc/articles/206544519).

The antivirus has been the same antivirus and IntelliJ has not taken time as much it takes now. Default settings have been used for the installation. What plugins folders can be excluded from the antivirus monitor?

You need to exclude the installation folder and the folders mentioned in the document I've linked above. Try on the machine with no antivirus and compare the performance.

I noticed the same thing, and yup, it's the Antivirus. I don't know who changed what but it's annoying now.

For IntelliJ IDEA to start and load fast, do the folders, mentioned above,  even today, need excluding from antivirus monitoring?

Yes, see these documents:

https://intellij-support.jetbrains.com/hc/en-us/articles/360005028939-Slow-startup-on-Windows-splash-screen-appears-in-more-than-20-seconds
https://intellij-support.jetbrains.com/hc/en-us/articles/360006298560-Antivirus-Impact-on-Build-Speed .