Edit: It doesn't happen on control panel pages with less content: like the Dashboard or Pages page. It happens on any pages that have a lot of content and therefore scrolling. Like editing fieldsets or editing an Entry with enough fields that the page scrolls. As soon as there is a lot of content on a control panel page, the entire cp crawls to a halt, sidebar becomes less responsive, fieldsets are slow to respond (like Select).
This didn't seem to be an issue before the recent control panel redesign...
Edit: I have tried it on Sierra and High Sierra, tried with a completely reset browser no plugins or extensions, in a local dev environment with fresh install, and a long-running live site. Using a 15" retina mbp display at full width.
To reproduce, use Safari on a mac at full 15" width; create a page with a handful of fieldsets, and on the page with fieldsets or even while editing fieldsets through the CP try and hover over sidebar nav-links. You will notice how unresponsive and laggy the hover-states are compared to on a dashboard or shorter page. This carries through to opening Select fields and many controls.