Let's do it quick!
Find your device's
swaymsg -t get_inputs.
Mine looks like this:
... Input device: TPPS/2 Elan TrackPoint Type: Mouse Identifier: 2:10:TPPS/2_Elan_TrackPoint Product ID: 10 Vendor ID: 2 Libinput Send Events: enabled ...
Test what value fits you:
swaymsg 'input 2:10:TPPS/2_Elan_TrackPoint pointer_accel -0.5'
Value is in range
0 is the default, so
-0.5 will decrease sensitivity by a half (whatever that means),
0.9 will increase sensitivity almost to its full.
Once you're good, just append the
input ... command to your
sway config for persistence.