Ok, I haven't had a chance to look at those videos until a few minutes ago, and I thought I'd give some reasons for which it appears slow, basign my judgments on what I see and what I know about the AmigaOS internals.
First of all, I must say that, although there was perhaps nothing more to see, those videos only show one little part of the GUI system, which, from what we know, could perhaps be the only one a bit flawed.
In essence, what we are shown are 2 things: the speed with which Reaction GUIs are drawn (in Solid-Resize and GUI-Reactivity), and the way (a faulty one - but read on) opaque window moving is implemented (in Opaque-Move).
Indeed, Reaction GUIs are drawn quite slowly, but I can't help but notice that Reaction GUIs have always been inherently slow, at least on my UAE setup, much slower than any other GUI, even MUI. The reason for this can be researched in the fact that Reaction uses a completely different approach than MUI, which is also the reason for which I greatly dislike Reaction in favour of MUI. However, it's obvious that the redraw is quite slow also considering Reaction's faults, and this is possibly due to the fact that the gfx library was emulated, along with a basic chipset support (some blitter thingies, like the BLTDONE flag in DMACONR register), which surely slowed everything down a lot.
Using this argument to say that AOS4 is at the same stage MOS was 2 years ago is pure flamebait for mainly 2 reasons: 2 years ago MOS already had a native gfx subsystem (which renders the comparision useless, although it might seem that it makes the situation even worse for AOS4), and also because this gfx subsystem is a temporary one, and we don't know at which state of development the new one is (but I reckon it's close to completion, or on the way to it). In any case, such comparisions are meaningless because AOS4 has now things which MOS didn't have 2 years ago, and vice versa.
About the opaque moving, instead, it can be noticed that there's a flaw in its implementation, which basically makes it *very hard* to *impossible* for the damaged windows to refresh themselves until the movement has stopped. Read Georg Steger's explanation of this phenomenon here.