@nortoniq: Typically, the Play Speed options at the editing stage should be visible by following the steps below.
Open the Gallery app > Tap the 3 lines in the bottom right > Videos > Select a video > Tap the pencil symbol at the bottom of the screen. From here, you should see a series of icons along the bottom of the screen, just above your Navigation Bar (Recent, Home and Back buttons). Swipe to the right until you seen 3 circles with a triangle in the largest circle (to the left of the musical note). Tap on the 3 circles to show your Play speed options above, make your selection, then tap 'Save' at the top of the screen.
Let me know how you get on.
@nortoniq: Have you tried swiping past the smiley sticker symbol, as more options should be available here? If not, try tapping the 3 vertical dots to the right of the sticker symbol to see if this produces further editing options.
I discovered, that somehow only videos which I shot with my phone camera can't be sped up. Something that worked for me: I changed the resolution down (from 2160x3840 to 1440x2560) and then saved that edited video as a copy. Now there is a option to change the speed of that "new" video.
I hope that helps.