Bug 1573202

What's the udevadm info output for the device please? udevadm info /sys/class/input/eventXX with the device's event node. And what's the output of xinput list-props <device name> for the trackpoint? Have you tried changing the pointer speed. if there's no toggle in the GUI, this command should change it:
   xinput set-prop "device name" "libinput Accel Speed" 1.0

allowed range is a double from -1.0 (minimum) to 1.0 (maximum)

Here is the original outputs (before speeding out manually):

I was little confused about device names between 12 and 13. Here is,
==========================================
# xinput
⎡ Virtual core pointer                          id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ Synaptics TM3053-004                      id=12   [slave  pointer  (2)]
⎜   ↳ TPPS/2 IBM TrackPoint                     id=13   [slave  pointer  (2)]
⎣ Virtual core keyboard                         id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
    ↳ Power Button                              id=6    [slave  keyboard (3)]
    ↳ Video Bus                                 id=7    [slave  keyboard (3)]
    ↳ Sleep Button                              id=8    [slave  keyboard (3)]
    ↳ Integrated Camera: Integrated C           id=9    [slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard              id=10   [slave  keyboard (3)]
    ↳ ThinkPad Extra Buttons                    id=11   [slave  keyboard (3)]
==========================================================
So, TrackPoint shows as id=13.
==========================================================
# xinput list-props 13
Device 'TPPS/2 IBM TrackPoint':
        Device Enabled (143):   1
        Coordinate Transformation Matrix (145): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000
        libinput Natural Scrolling Enabled (287):       0
        libinput Natural Scrolling Enabled Default (288):       0
        libinput Left Handed Enabled (289):     0
        libinput Left Handed Enabled Default (290):     0
        libinput Accel Speed (291):     0.000000
        libinput Accel Speed Default (292):     0.000000
        libinput Accel Profiles Available (305):        1, 1
        libinput Accel Profile Enabled (306):   1, 0
        libinput Accel Profile Enabled Default (307):   1, 0
        libinput Scroll Methods Available (293):        0, 0, 1
        libinput Scroll Method Enabled (294):   0, 0, 1
        libinput Scroll Method Enabled Default (295):   0, 0, 1
        libinput Button Scrolling Button (308): 2
        libinput Button Scrolling Button Default (309): 2
        libinput Middle Emulation Enabled (299):        0
        libinput Middle Emulation Enabled Default (300):        0
        libinput Send Events Modes Available (264):     1, 0
        libinput Send Events Mode Enabled (265):        0, 0
        libinput Send Events Mode Enabled Default (266):        0, 0
        Device Node (267):      "/dev/input/event12"
        Device Product ID (268):        2, 10
        libinput Drag Lock Buttons (303):       <no items>
        libinput Horizontal Scroll Enabled (304):       1
=================================================

This seems to show Accel Speed as 0.0. Now there is no event13 but event12
seems to be the one.

==================================================
# udevadm info /sys/class/input/event12
P: /devices/rmi4-00/rmi4-00.fn03/serio3/input/input17/event12
N: input/event12
E: DEVNAME=/dev/input/event12
E: DEVPATH=/devices/rmi4-00/rmi4-00.fn03/serio3/input/input17/event12
E: ID_BUS=rmi
E: ID_INPUT=1
E: ID_INPUT_MOUSE=1
E: ID_INPUT_POINTINGSTICK=1
E: LIBINPUT_ATTR_TRACKPOINT_RANGE=50
E: LIBINPUT_DEVICE_GROUP=11/2/a:rmi4-00
E: MAJOR=13
E: MINOR=76
E: POINTINGSTICK_CONST_ACCEL=1.0
E: POINTINGSTICK_SENSITIVITY=200
E: SUBSYSTEM=input
E: USEC_INITIALIZED=6091684

=====================================================

After doing

xinput set-prop 13 "libinput Accel Speed" 1.0

The mouse speed becomes 1.0 and speed returns to normal

I hope this helps but I hope I don't need to put this into rc.local
but can be set automatically.

X device IDs and event nodes don't match up, so the event node you picked is the right one. Luckily the T450s is one device I have access to so I tested with with F27 updates all applied.

And the pointer speed for the trackpoint is as expected. setting the accel to 1.0 makes it almost unusably fast (again, as intended).

udevadm info is exactly the same. What's the output of cat /sys/devices/rmi4-00/rmi4-00.fn03/serio3/sensitivity (fix any typos where needed please). Should read as 200 based on POINTINGSTICK_SENSITIVITY=200. If that is lower then that would be the reason why the trackpoint is slower than expected and we have to figure out why systemd didn't set this on boot up.

Hi, I am not an expert but under KDE plasma trackpoint accel 1.0 works great for me it is not jittery at all. I never had problem with the trackpoint and I exclusively use that. I turn off trackpad in bios (if I remember correctly).

accel 1.0 is the maximum setting and it's deliberately set too high, both to catch odd corner cases but also to indicate that if you ever need 1.0, something isn't quite right.

Again, what's the output of 
cat /sys/devices/rmi4-00/rmi4-00.fn03/serio3/sensitivity

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