idk what morph or what usecase...but for single morph with random speed i would just use builtin animation pattern to control morph
(using animation triggers [transition action] 1 for A>B and 1 trigger for B>A)
for speed change i would just load included FloatParamRandomizer plugin on animation pattern atom (randomize animation speed)
however...if you don't wanna add extra animation pattern atoms inside scene
you can use builtin scene animation
-record few secs empty animation than cut it (to your desired time length), add same animation trigger (using transition)
load floatparam plugin (anywhere..inside scene plugins is fine) and let it randomize coreControl/motionAnimationMaster/playbackSpeed
same effect, less stuff on screen
no need to overcomplicate stuff, those are 2 basic ways i can think of...
ofcourse for anything more complex you should use timeline
attached quick example scene controling mouth open morph (using both AP and scene animation with random speed)
EDIT: answer to title, you can also control UI slider value with triggers (UIslider/Trigger/value)