As others like Johnny Oldenburger have blogged, Notes 9.0.1 FP10 IF2 is out now. This fixes a number of Designer-related issues, including the problem with the Display panel in the Java debugger I encountered when I first installed.
One point to be aware of is, as ever, to check <Notes>\framework\rcp\deploy\jvm.properties for the vmarg.Xmx and vmarg.Xms settings (the de facto starting point for any “Notes is slow launching” issues). My starting setting (Xms) was reset. A quick update again and the world is good!