On a Lenovo ThinkPad X280, I observed that Plasma 5.27.11, upon the first startup, set the mouse (as which I assume TrackPoints also counted, see: https://bugs.kde.org/show_bug.cgi?id=463825) acceleration profile to "flat," making it painfully slow even when dragging with full force.

Running `xinput list-props` on the TrackPoint devices yields the following regarding libinput acceleration profiles:

libinput Accel Profiles Available (342): 1, 1, 1
libinput Accel Profiles Enabled (343): 1, 0, 0
libinput Accel Profiles Enabled Default (344): 1, 0, 0

Which, according to libinput's latest documentation, meant that all three (adaptive, flat, and custom) profiles are available to the TrackPoint, with "adaptive" enabled as default on this device. However, upon checking ~/.config/kcminputrc, we see:

[Mouse]
X11LibInputXAccelProfileFlat=true

Contradicting with libinput's defaults. I can assure that I have not altered this setting via neither xinput(1) nor any Xorg configuration files, since the acceleration profile is correct in SDDM and was only set to "flat" upon login (I have also tried setting the "flat" profile in an Xorg configuration and the TrackPoint became painfully slow in SDDM as well, so I think my findings are consistent).

I have also pin-pointed the issue to commit fb8a655a7376 ("If no config option for accel profile is selected, use default from Xorg") - reverting which fixed the issue for me, but obviously, that was due to a hard-coded "adaptive" profile. So, it would appear that the X11LibinputDummyDevice::getDefaultConfigFromX() function return-ed in the middle, before the flat profile property could be set to "false" (and fell back to "true"), hence this issue.