I'm on mac, but opened up my windows laptop to check this. I clicked the start menu, typed "mouse speed" and the first option was "Change the mouse pointer display or speed", I picked that and there I was.
The main issue is that a search for "mouse" or the like takes you to the new Settings panel for mouse instead, which has some options but not all. A small "Additional mouse options" link under a "Related settings" heading then takes you back to the old Control Panel mouse settings, where you can actually change the speed.
Your search was good, and got you straight to the old Control Panel version. But the fact that the settings are split over two not-fully-overlapping systems is a bit ridiculous, and confusing.