Not sure when this changed, but felt that dragging a selection felt a bit laggy with the latest master. Checked out an old arbitrary revision, and current master feels like a 10x performance degradation (or at least one order of a magnitude), very visible when e. g. starting a selection and dragging upwards into the backlog. Probably bisectable, so I'll try to find some time to do it unless someone knows what is going on.
two things on my mind 1 - the new visual selection behavior *could* do this 2 - the reflow code could potentially have done this too.
Seems like there were some minor slowdowns from other stuff (probably the stuff you mention), but the thing that made it unusable, at least when scrolling, is 6667d96ed9d6c95eab4ed13449d4b134005ffb4b (was a quick bisect)
The commit message for that seems bogus as well, 6667d96ed9d6c95eab4ed13449d4b134005ffb4b^ works just fine in normal vim (don't have neovim, though, but then it sounds like a neovim bug).
A possibly relevant merge request was started @ https://invent.kde.org/utilities/konsole/-/merge_requests/404