[DiapDealer]

Are you testing with Sigil's default empty epub, or an epub of your own? I'm asking because I'm trying to determine if the epub itself might not exacerbating the issue. Giant epubs (especially ones with very long/giant xhtml files with tons of images) can cause Sigil performance issues in general.

In all of my testing (on about four different machines--one of which has an AMD processor), I've noticed a bit of memory increase when minimizing/restoring, or resizing Preview/Sigil, but nothing that approaches the levels you're seeing. It takes me forever to get memory usage increased to 1Gb. And in all cases, disabling hardware acceleration for QtWebEninge in Sigil's preferences stops the problem completely (after restarting Sigil). With no noticeable performance loss. But I also don't edit/create/open epubs that I know will cause performance issues for Sigil. That might be the difference.