Looking around it seems like the accelerated Shift + arrow keys movement default is 25 pixels. Somehow mine seems to be 2 pixels. Can this be customized?
The arrow keys move 1px by default. Holding down Shift moves in bigger increments, but the amount depends on your zoom level.
If you are zoomed into the pixel level, Shift+arrow keys moves in 2px increments, but if you zoom out if moves in progressively bigger increments. Makes sense if you think about it.
I think this behaviour was changed in newer versions. However it doesn't seem to be documented on the GIMP Docs page yet. Maybe it needs to be updated. I only discovered it by testing.
As far as I'm aware, you can't change this behaviour other than by zooming out. If there was a way it would probably be in the Preferences, but I couldn't find a setting for it.