The teams have very different visions. Basically incompatible at a core level. From the very basic UI libraries. KDE uses QT, a pretty open source friendly tool kit, but IIRC not actually open source itself. Gnome uses GTK. The g in GTK originally referring to Gimp.
Gnome itself has traditionally had a much more apple way of doing things. If you venture outside of that. Things can break easily. And at a base level there are things gnome is kind of reluctant to do UI wise. That said, it’s simple and works well. KDE is kind of jack of all trades master of none. But it still does. All them quite well. If you want to mimic Mac OS. You absolutely can right down to the theme panel placements etc. If you want to emulate windows it’ll do that perfectly too. If you want to look like gnome. Yup. If you wanted to look like some unholy abomination the world was never meant to see. They got you covered as well. In fact, I think Garuda Linux comes default with that.
However, choice is never a bad thing. And there’s no real reason for them to merge. They run side by side. Quite brilliantly. And I even find myself popping back and forth from one to the other randomly. Your issue with snappiness under KDE could be a couple of things. There are a lot of configurable settings under the hood. You can decrease animation times etc etc etc. Which will often give a feeling of snappiness things just moving faster. But at a more fundamental level. Gnome has been a bit better optimized for use under Wayland etc. Which most distributions are moving towards. Which greatly increases the performance and responsiveness of many things. KDE has been lacking in at department. Though early this year we may see a solution to that. In the meantime though, sit back relax. Enjoy both and compute how you like.