Ain't that the truth. I don't know why KDE Discover is so sluggish when it comes to Flatpak, it takes me like 10+ seconds to load the landing page and see the popular apps.
First time I've heard someone call Gnome Software fast. In my experience that app feels like it's on it's last legs; the Flatpak CLI is far better than any desktop GUI.
Gnome Software has received numerous updates over the last few years which make it considerable faster. Searching and viewing apps is now fast enough to be usable, compared to it taking many seconds to minutes for basic tasks.
I've stopped removing Software on every system, altough I'm not usually using it. I've not tested it, but I feel like Discover is now slower than Software.
My main issue with Gnome Software is if I queue something to install, and go back to browse for more apps, once something is done installing it "refreshed" and I lose the spot I was at. Makes me feel I can only install one thing at a time.
It's definitely faster than it used to be. But yeah, searching for app updates is still more sluggish than through the terminal, at least on Fedora Workstation.