You’re misunderstanding what the speed knob is for. The speed knob only affects the display in the RTA and Phase views (the second and third settings of the “Mode” knob), and it affects how rapidly those displays respond by changing the decay time of the display.
In order to make the waveform display “slower”, you need to have it displaying a larger segment of time so that it takes longer for a given sample to traverse from right to left across the display area. You adjust that with the sliding blobs underneath the waveform display, above the knobs. Sound is being generated as fast as it is…you can’t make it “slower” without changing the playback rate. If you could actually slow down the display without slowing down the sound, it would rapidly get out of sync with what you were hearing, making it less than useful.
Hope that helps.