Spinners have inconsistent/variable speed
Testing Fedora 30, spinners rotate at an inconsistent speed - they seem to fluctuate between moving too fast and too slow. Here's a recording of it: Peek_2019-03-12_18-27.
That's from gtk3-widget-factory (GTK 3.24.5), running in a VM. You can see the same behaviour in apps, such as Software.