I've found a partial workaround (the old version still behaves better), I don't know if it could be useful to find the problem: In KDE System Settings -> Input Devices -> Touchpad -> scrolling -> Move distance to scroll a single line -> set to 25-30. On my machine the touchpad returned partially functional, scrolling at a slow speed remains very difficult.
full configuration: $ synclient -l Parameter settings: LeftEdge = 1752 RightEdge = 5192 TopEdge = 1620 BottomEdge = 4236 FingerLow = 25 FingerHigh = 30 FingerPress = 256 MaxTapTime = 180 MaxTapMove = 221 MaxDoubleTapTime = 180 SingleTapTimeout = 180 ClickTime = 100 FastTaps = 0 EmulateMidButtonTime = 75 EmulateTwoFingerMinZ = 282 EmulateTwoFingerMinW = 7 VertScrollDelta = 30 HorizScrollDelta = 100 VertEdgeScroll = 0 HorizEdgeScroll = 0 CornerCoasting = 0 VertTwoFingerScroll = 1 HorizTwoFingerScroll = 0 MinSpeed = 1 MaxSpeed = 1.75 AccelFactor = 0.1398 TrackstickSpeed = 40 EdgeMotionMinZ = 30 EdgeMotionMaxZ = 160 EdgeMotionMinSpeed = 1 EdgeMotionMaxSpeed = 401 EdgeMotionUseAlways = 0 TouchpadOff = 0 LockedDrags = 0 LockedDragTimeout = 5000 RTCornerButton = 0 RBCornerButton = 0 LTCornerButton = 0 LBCornerButton = 0 TapButton1 = 1 TapButton2 = 3 TapButton3 = 2 ClickFinger1 = 1 ClickFinger2 = 1 ClickFinger3 = 1 CircularScrolling = 0 CircScrollDelta = 0.100007 CircScrollTrigger = 0 CircularPad = 0 PalmDetect = 0 PalmMinWidth = 10 PalmMinZ = 200 CoastingSpeed = 0 CoastingFriction = 50 PressureMotionMinZ = 30 PressureMotionMaxZ = 160 PressureMotionMinFactor = 1 PressureMotionMaxFactor = 1 GrabEventDevice = 1 TapAndDragGesture = 1 AreaLeftEdge = 0 AreaRightEdge = 0 AreaTopEdge = 0 AreaBottomEdge = 0 HorizHysteresis = 25 VertHysteresis = 25 ClickPad = 0