I'm just having a detailed look at the behaviour of LinuxCNC on my system and noticed that jogging via keyboard is not smooth - and this seems to be unrelated to my settings.
G0 / G1 is smooth for the same path with the same feedrate.
So it seems like the simple motion planer is repeatedly running the de-acceleration / acceleration for a short time when it receives jogging requests, at least there seems to be a gap between the requests.
The issue can be covered a little bit by setting the acceleration a little bit slower in the ini file - but it is still there.