First, replace the core GNOME apps you use most frequently with lighter alternatives. Take it easy, do it one at a time. Try rxvt-unicode instead of gnome-terminal. Try Thunar or Dolphin or an mc clone instead of Nautilus.

There's a lot of non-obvious configuration options in the X11 stack. An .Xresources file can select better fonts, tweak hinting so they look fantastic, and change settings and color themes for many X applications. Out of the box, many X applications look like obsolete garbage, but they can be lightweight, functional, and awesome looking at the same time with a little configuration and love.

After getting your core apps sorted, consider an alternate DE/WM. I personally love the gaps fork of i3. XFCE is nice if you still want DE functionality.

I personally dual boot Debian Stretch and Gentoo ~amd64 - sharing a home partition. I keep Debian more or less stock on GNOME, while I rice the hell out of Gentoo with i3 and no systemd.

Check out https://reddit.com/r/unixporn for inspiration and ideas.