"i7 CPU" is totally zero informative for now. It's vary from 2core 1GHz (ancient i7 620UM) to 18 core.

That is strange. Until fairly recently my daily driver was a low end ex-lease HP machine from 2013 or so with an i3 with 4gigs of RAM. Never saw much issue with Defender or the updates. Not unusual for updates of any kind to be all over and done with in <5mins from clicking update to a fully usable desktop. Win10 has improved a heck of a lot over previous versions even on old hardware. My work-supplied Mac on the other hand, even with much better specs than my lowly home machine, usually takes at least 30mins to do updates (with most of the time spent during the reboot which means I can't use it at all). One of many reasons why I moved away from Apple for my own hardware.